I'm planning to use my miles to upgrade a vacation next summer, but don't know where I'm going yet. I know that upgrade seats become hard to find close to high season for many tourist places (SYD, etc), and was wondering if anyone knows of a list of regions of the world with their corresponding high/low tourist seasons.

Something like:
South Pacific - High (May-June), Low (...)
Western Europe - High (...), Low (...)
...
...

Thanks for any info.

While I don't think Hawaii ever has a true "low" season, my wife and I take all of our trips there during May-Sept so that we can use points and miles for the trips. We've done 4 weeks in hotels using MR and HH awards, and have redeemed UA and AA miles to get there. We were able to pick & choose our dates for the most part - we usually book in January.

Added bonus: none of our hotels were terribly full, so we received great rooms on all of our award stays (not suites, but high-floor full-ocean-view rooms). Also, booking activities and tours was fairly easy to do at that time of year - we didn't need to prebook luaus, boat rides, or tee times weeks in advance. Just make phone calls a day or two beforehand to arrange the activities...

As for Western Europe, I think the airlines consider May 15 - Oct 15 as the high season. The big hotel chains are probably similar... However, I have spent time all over Europe in September and loved it: I didn't feel that September was a bad month to be there at all. (I get the sense that max-peak tourist season is more like July-August.)

I have been to Europe in May and early June as well as Sept., and feel it is also a good time to travel -- not very crowded, pretty good weather, etc. It may be considered high season with respect to prices, though -- I can't be sure about that. I agree that July-August is definitely the highest of high season in Europe and should be avoided if at all possible.

True - I've done short trips in May as well and found reasonable prices. I think it's high-season w.r.t. the mileage programs and probably some airfares, but deals can still be had...

It all might be how/when you time your trips. There are even possiblities during the summer for decent fares to Europe. Last summer, I flew RT to Ireland , and got upgrades both ways (using miles) to business, and day-long (23 hours!) layovers both ways for times to explore London---during summer--for a farely good price. I flew out the last day in early-mid june before fares went up, and flew back on a low-travel day around the 4th of July holiday.
Or, your keen on the south pacific area---last summer, Quantas, Air Tahiti Nui, and Air New Zealand put r/t's there for half of what you'd pay during their "high season" (our winter, their summer).
Search F/T for "Site 59" and you'll see some of the GREAT deals that F/T'ers got this past summer going to Europe, if you're able to be a little spontaneous about where you'll go.