Yes my macbook pro early 2009 unibody (A1297) had a battery swelling problem. It forced the trackpad higher and bulged the bottom of the unibody case.
Just happened with age. Cant tell you when it started cos I bought it like that.
If you take it into apple, and tell them about it, they might replace the battery for free.
They did with mine and that fixed the trackpad and case bulged. brand new again. :-)
FYI: I didnt tell them I bought it second hand. I also did tell them that the problem is a well known issue
which it is. They know this. Make sure they check the history on this macbook pro. The genius guy nearly charged me until he continued reading on his ipad.