I usually edit gifs using CS3 but then I upgraded to Lion and my CS3 wont seem to import gifs anymore (using video frames to layers option). even when I typed xxx.gif it wont let me pick my gif.
However, I went to a machine here that still has a copy of CS3 (and is running Snow Leopard, by the way), and was unable to open the .gif file and keep the animation frames.
If you have the CS3 suite, you should be using Fireworks for animation. It will be better suited to that task.
I'm not sure how you got it to work before, but as far as I can tell, it doesn't work under 10.6 or 10.7.
CS2 and other earlier suites also contained ImageReady, which worked great for things like animated gifs. Obviously, CS2 isn't an option under Lion, however.