How water resistant is the HTC Desire?
If the water finds its way into the circuitry and causes a short, that could be bad, but I don't think there is any good way to quantify how likely that is to happen to any phone.  I have a Moto Droid and get it a little wet all the time and have never had a problem, although I have heard of people bricking their phone under the same circumstances.  

If you do actually get it wet and it dies, it may not be bricked.  After it dries out it will probably work again.  You can aid the process by putting the phone in some rice or just blowing dry warm air on it.  

It's also worth noting that many phone manufacturers install moisture detecting stickers inside the phone.  On the Droid it can been seen through a little hole under the battery cover that shows red plus signs on a white background.  If that gets wet, it'll bleed and that way if you try to return a broken phone and say it just died the tech can see if you got the phone wet, and even if that's not why it died, it'll void your warranty.