Same here
Just wanted to chime in... I've been noticing the exact same behavior with my 4S and Hipstamatic since the day I got the new phone. I never saw anything like that with my 3GS, and I was using Hipstamatic a lot.
I don't have any issues with other camera-apps though, I can take 40 to 50 pictures in a row before the phone gets even slightly warm - but with Hipstamatic it gets unbelievably hot after just a couple of photos.
Yesterday I found a couple of "Low Memory" logs under Diagnostics & Usage on the iPhone - the largest process and the only active application at those times (according to the logs) was Hipstamatic. Weird.
Anyway, I'm kind of glad to see I'm not alone - makes me think it's a problem on Hipstamatic's end.