iPhone 3Gs got dropped in water. Is it pretty much done?

When this happens don't turn the device on or try to sync it right away. Instead, remove the battery (if possible) and put the device into a sealed bowl that's half full of uncooked rice, crystal cat litter (make sure it's pure crystal) or the desiccant crystals often sold in garden centers. These will all draw the moisture from the device and absorb it.

Leave the device in the sealed bowl for at least 24 hours. 48 is better, but you might want to check/change the rice after 24. No need to bury the device either--just rest it on top or suspend it on a block of wood, a couple of dowels, or whatever else you can one up with. After 48 hours inspect the device. If you think it's dry go ahead and test it.

No guarantees, but with luck you'll be pleasantly surprised.

Actually, water is not the issue; it's the contaminants in the water coupled with long drying times in enclosed spaces with narrow gaps that does the most damage due to corrosion.

To be completely successful against corrosion, you need to CLEAN or flush the contaminants out. Working against this is the competing problem of water damaging some of the elements like the battery or causing an electrical short.

Remove the screen and LCD off your 3GS, and place the rest of the phone (without screen) in 99% isopropyl alcohol. Alcohol will damage the LCD, so that's why the removal is necessary.

Alcohol is NOT conductive (neither is pure water), it will chase the water & debris out (ever had swimmers ear?) and evaporate much faster.

Works every time, as long as you didn't plug in or keep turning in the phone prior.

If it's still dripping wet and the water it fell in wasn't clean (or worse, was salty), and you have distilled or deionized water handy, don't turn the device on, but promptly redunk it in distilled water to flush out any contaminants before they start drying on the internal circuits.

Then leave in a bag or container of desiccant (uncooked rice is common) in a warm (not hot) place (windowsill, etc.) for a few days. You can wrap it in a handkerchief or paper towel to keep dust out of the ports while in the desiccant container. 48 hours may not be enough, some people have reported their device not coming fully back to life until around a week or more later.