Battery won't budge - suggestions?

I'm trying to replace the battery on an iPod 5G video (30GB). I got the case open (after some drama - I think the plastic got brittle over the last eight years...) but the battery won't budge. The directions say that the battery will come loose with some prying, but mine seems fused to the case. I'm afraid I'm starting to bend the case itself, and I'm worried I'll damage the ribbon cable that goes under the battery.

A friend suggested using a heat gun to melt the glue, but using a heat gun on a battery seems like a bad idea.

Ideas?