I guess the title says it all.

Since the firmware update, everytime I charge my cellphone, my N900 gets hot. Like seriously hot. I end up making sure I don't let it charge continously for more than 30 mins to prevent from overheating and getting the back cover melting :P

Anyone else having this problem??

mhm... could you have a look at the cpu-usage while loading?
via htop-app or top in xterm?

No I dont have it, sounds like

A: hardware failure of the charge mechanism not doing what its supposed to

or

B: it was software/firmware controlled and now its malfunctioning.

I would flash back to stock firmware right away and see if it fixes it, if its B that should fix it, and if its not B then atleast you will know its A and can start the process to get a RMA and a new phone.

Leaving it like that is probably going to be bad news for your device at some point.

Forget the device, I'd be more concerned about Fire if it really is getting that hot. As said, I would reflash. If it's still acting up I'd take it to a nokia center ASAP.

There was that problem with batteries overheating and exploding, even iPhones have been doing it when getting too hot. It it is hot and isn't just warm, I'd get it looked at ASAP and would advise not using it.

my N900 gets warm, but I'd never describe it as hot. It's passive cooled but no way on earth should it be getting hot hot.

i would say that either one process is running amok and causes 100% cpu-usage which will heat up the device...

..or (as vicious mentioned) there is a software problem (phone has problems to control charging) or a hardware problem.