I've got an iMac, and when the battery died, nothing went wrong really, it just forgot the date and time and other stuff when it was disconnected from the wall.
If you have a multimeter then it should be easy to take the battery out and check it yourself.
I bought a new one at RadioShack, and for an extra $5 or so I got a 3 year warrenty on the battery, so I can bring it in if it's dead and they'll replace it free for the next 3 years.