I like the idea of a down bag, too. But I cant help but think the humidity will pretty well kill it. So Id go with synthetic. Its not as if youre going to save a lot of weight by going with down, anywaynot in a bag rated around 40 degrees.
You could, for instance, get a Mountain Hardwear UltraLamina 45-degree bag ($170). The name is the temperature rating; its strictly a warm-weather bag, but likely would be just the ticket for you. Its fill is a proprietary polyester material, which is a good thing because polyester is reluctant to absorb much water so will stay dry. It weighs a pound and a half, so it wont make a big dent in your luggage allotment. The Sierra Designs Yachiyo 45 ($109) is very similar in weight and temperature rating, and its also considerably cheaperperhaps due to fill that isnt quite as durable. But it would work well for you.
You can add some warmth to either bag, or have a standalone sleep-shell for very warm nights, with a Cocoon Ripstop Silk Bag Liner ($65). Id recommend one in any event, as using an easily washed liner will help keep your main bag cleaner and extend its life.
And, for a warmer bag, look to REIs Nooksack +35 UL ($170), a bag that uses Primaloft insulation, a material I like a lot because its soft and down-like, yet resists moisture extremely well. It weighs just 28 ouncesnot bad!
So there you go. Hope you have a great trip!
The 2008 Summer Outside Buyers Guide is now on newsstands. Look for it online soon.