I don't know many consumer devices that I would recommend removing the battery while still connected to a power outlet. I once bricked a Mac PowerBook doing that very same thing. For me, personally, I'd never try it again with any device.

I'm not saying that this actually HAS bricked your 800, just that it is possible. Like I said, try leaving it unplugged for an extended period of time and see if that helps.

Maybe someone else will drop by, see this thread, and be able to give you some more authoritative advice.

I seem to remember having a similar "Panic Mode" incident right after I got my n800, in which the device wouldn't boot, but can't remember the exact combination of steps that I took to get the device back up. I think it may have just been a matter of leaving it alone for a long time, or letting it charge for a long time. Either way, after that first "OMG!" that happened in the first 24 hours of ownership, I've never had a problem since.