My body and lens do not have weathersealing, but I don't take it in the conditions you will. I would buy one if I were you. If your worried about price, then take a look at the new Pentax cameras, they are all weathersealed. The K20 was rated very highly in reviews. I never pimp out Pentax, but this might be a good choice for you.
Here's a review of the K20
I'm a hobbyist looking to go much further during the next couple of years, and trying to decide on my next step up dslr, and it's between the Nikon d90 or d300.
Now, I like the price of the d90, (especially with the economy as of late!) and am sure that I won't out grow it for a long enough period of time to be a worthwhile purchase.
However, I'm living and traveling in Asia and SE Asia, and am touring from the sands of Mongolia to past Cambodia, much of which will be during respective monsoon seasons, sometimes in the jungle.
Bearing these climates and conditions in mind, is a weather and dust sealed model such as the d300 (with it's 18-200 kit option) a must?
That's a big step-up in price.
My gut is telling me yes, my wallet however, is singing a different tune.