That must be something fairly new, but certainly good to know. Price is a bit steep, but far cheaper than 1gb on most roaming plans. Even living here, data is all I use 99% of the time, I may make 1-2 voice calls every month or two.

In that case if going the B-Mobile route, I looked at their website and it looks like it's available at a few of the Yodobashi Camera stores, you can usually find a couple people who speak English there to help you. I found another website that may be useful for you, seems several companies have jumped on this:
http://www.nippon.com/en/features/h10011/

You'll be able to get one type or another at Haneda when you land and it should be priced roughly the same as downtown, would be worth it if you don't know exactly where your hotel is located, that way you can access Google Maps when you're looking for it.

As another poster mentioned, carrying your bag through major stations can be challenging because of the crowds, on weekends the major stations get quite crowded as do the trains. I do it all the time with a 28" bag plus my carry on in the other hand after a shopping trip to the US and frankly don't care but it could be very frustrating for someone unfamiliar with Tokyo. Some stations may not have an elevator or escalator to the platform, this is especially bad on Metro lines, so depending on where you're going you may have to be prepared to carry your suitcase up and down stairs. If the Limousine Bus goes to your hotel or very near, I'd seriously consider doing that over taking the train/monorail from Haneda then switching to other lines. If you do end up taking the train then the shipping option from the airport to your hotel would be nice, just take out the essentials you need that day and your bag would arrive sometime the following day. I think it runs about 1,500 yen (forget exactly, as I get it free with my credit cards). You'll see the counters at Haneda to the right just as you exit customs.