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Without knowing exactly how warm it is it's difficult to say. Being a little arm is ok, but you don't want it getting hot. That could mean a few different things; bad battery, bad cpu, or a badly coded app that you've installed.
If the tablet is getting really hot with just average usage; make both an encrypted backup to your pc/laptop using iTunes and also an encrypted backup to iCloud. Then factory reset the tablet and set it up as new.
Before re-installing from backup run the tablet for a day or two doing the same thing you were before when you noticed it heating up. If it gets too hot again then make an appointment with the Genius Bar at your local Apple store. Excessive heat after a fresh reset running nothing but default apps may mean there is something wrong with the hardware.
If it doesn't get hot then you know it was a bad app, and you'll need to figure out which one it is so you can file a bug report with the app developers.