Well, since PS CS3 doesnt include imageready, many people have problems importing animated .gifs as layers into Photoshop. If you try to open animated .gifs via File - Open only one frame is imported. With a little trick, it's possible to avoid this. Before you start, make sure Window - Animation is activated. Ok, start off with:
File - Import - Video Frames to Layers
Now you might say "but it says I can only import .mov .avi .mpg .mpeg". To see your .gif files, type *.* where it asks for the files name and hit Enter. You should now be able to see all kind of files. Select your .gif and click load
A new window appears. Make sure 'From Beginning to end' and 'Make Frame Animation' are ticked. Hit Enter or click OK.
As you can see, it opens all of the .gifs layers. You can choose between them in the Animation Window we've activated before. PS doesnt import the frames delays, you got to adjust them.
Once you're done with your editing and stuff, you can save your animated .gif by clicking File - Save for Web & Devices.. Choose your preferred options and click Save.