First of all, it's *very* bad, that You tried to charge it/turn it on. First thing You should do is removing battery... Still, You can try to disassembly it (many videos on net - youtube for example - process is easy), then put in bag of rice and warm, well ventilated place. Doesn't hold Your breath, thought.

By the way, being "modern" doesn't make phones (any device, in particular) waterproof. Even best short-circuit prevention system can/will fail, if You short every element with each another (what is basically thing *possible* when You cover it in electrolyte), not to mention oxidization etc...

Some people successfully resurrected their devices after diving deep in water while turned on, but they didn't tried to kill N900, by charging it and turning on while wet inside.

// Edit

Ops, too late. Still, *doesn't* try again tomorrow, cause it *won't* be dry inside, if You don't disassemble it as much as possible. Taking off only backcover isn't enough.