I agree with m-man that we all have different approaches, and if the OP wants to at least start thinking about where to go, there's no harm in that. Planning a trip like this can be something of a research endeavor, especially given the details that some of us here on FT (myself included!) like to delve into.

Anyway, OP, here are some initial observations and suggestions:

1. As folks have pointed out, there may be some great Starwood properties opening up that you'd want to read reviews of. Nevertheless, I'd very, very strongly recommend that you consider Le Meridien Khao Lak, about a 75 minute/$40 taxi drive north of Phuket (and its airport). My wife and I were there a little over a year ago for 11 nights. Beautiful property, extremely long stretch of pretty and not very developed (so far) beach, delicious and cheap beachfront eateries just a 5-10 minutes' walk away so you're not stuck with hotel food, possibility (though not certainty) of using your points for a suite rather than just a regular room, lots of other activities to do in the surrounding area (if you're not lazy slugs like us), etc., etc., etc. The nicest beach vacation my wife and I have had, and we've had quite a few. Though the FT search engine can be quirky, you should be able to find the thread dedicated to this hotel by simply punching in the words Khao Lak. If you don't want to go through the entire thread (though it's worth it, to get various perspectives), go to my post #163, which has my own opinion and an excellent review by Sam P. Goodman. And feel free to pm me about this if you wish.
2. As for other places to go in a trip of up to three weeks: Though technically not in SE Asia, Hong Kong is a fantastic city to stop off in on the way to or from Thailand (preferably on the way to Thailand, since you'd be on the busy side in HK and then can relax more in Thailand) and you can get beautiful weather there in January or February (as opposed to very hot or muggy later in the year). We spent four days there on the way to Khao Lak--then flying directly from Hong Kong to Phuket on Thai--and loved it. Stayed in the Conrad rather than a Starwood property there, so can't provide info re Starwood hotels.
3. I've read good reviews about the relatively new Starwood (Le Meridien?) property up north in Thailand in Chang Rai. It would be a real change of scene, culture and cuisine from the south. Also, since its in the hills, I believe you'd get a change of scene in terms of the temperature also--very nice but springlike, as opposed to tropical down south.
4. Yet another place to consider is Siem Reap in northern Cambodia, where the Angkor Wat and other ancient temples are located. I know there is at least one Starwood property there, but don't know much about it. You can fly directly in and out of Siem Reap from Bangkok without going through Phnom Penh. It's a short and fairly cheap flight. Though I've been to PP and other places in Cambodia numerous times for work and once as part of a vacation with my wife--and I kind of like PP because it's a more manageable city than many other Asian capitals--the only place in the country I'd really recommend for your visit would be about two full days/three nights in SR. For example, the beaches, beach scene and quality of service in Cambodia just can't rival those in Thailand.
5. Folks rave about Luang Prabang (sp?) in Laos. But I haven't been there and don't think there are Starwood properties there.
6. Bangkok itself might be worth two or three days (at most)for a visit, as there are some very nice Starwood properties there, especially if you like big, bustling cities. I'd recommend the Starwood hotel by the river if this is your first time there, since its a pretty setting. OTOH, if you've been to Bangkok before or simply prefer less urban settings, I'd suggest just passing through to the extent necessary for other travel. And if you're going to spend time in even one city on this trip, I'd strongly recommend Hong Kong over Bangkok.
7. If you consider other places in SE Asia, be aware that climate can vary within the region. While January and February are the driest and (relatively) coolest months in much of the region, in much of Indonesia for example they are the rainy season.
8. You may find that you won't be able to book your Starwood rooms with points until 2011 because the award inventories may not open up until then. And while I infer from your post that you have hotel points but not airline miles, if you in fact are burning miles as well then for most or all airlines you won't be able to book award travel until 330 days in advance.
9. If you really have buckets of Starwood points, as you may know you can convert them into airline miles at a ratio of 20,000 points = 25,000 miles on a number of carriers such as AA (though on UA the conversion rate is worse). You can in turn then use the AA miles to get, for example, Cathay Pacific flights to the region. Or perhaps you can convert directly to Cathay Pacific--I don't know about that and don't know whether that would be the best option. You'd need to do some further research to find out which might be the best options if you decide to do this conversion.
10. If you decide where to go before booking your flights, that of course can come in handy in getting the best tickets available. If, for instance, you follow my great advice and go to Le Meridien Khao Lak, you'd want your paid or award tickets to include the trip to Phuket if possible rather than simply getting yourselves as far as Bangkok. And for that matter, as with our experience of going straight to Phuket from Hong Kong (instead of through Bangkok) or to Siem Reap from Bangkok (instead of through Phnom Penh), you might find that for some destinations you wouldn't need or want to go through a country's capital city.
11. I'd think that for most destinations you'd be better off in February rather than at least the first half of January, which tends to be a very busy time. Also, for some destinations you'd want to find out (and avoid) when Chinese New Year or other holidays fall, since they might be more crowded then due to schools being on vacation, etc.
12. Having waxed so enthusiastic about parts of Thailand, I'll add one final note of caution: the country could face political instability in coming years. Whether that will happen and whether it would impact your vacation (as it did in late 2008 when demonstrators seized the international airport and disrupted lots of visitors' ability to enter or leave the country) is up in the air. Personally, I find the chances of things getting bad enough to disrupt travel again precisely when anyone is taking a specific trip pretty low (not that I'm a Thailand expert), and these considerations would not stop me at all from planning a business or vacation trip there. But everyone has different risk tolerances, and you should be aware of these possibilities.

Hope this helps!