If you are using CS4, its pretty easy, pull all the images into PS, on seperate layers, and open the ANIMATION panel.
In the animation panel, you open up the set number of frames you want.
Select the first frame, and turn off EVERY layer, leaving only the first image layer on.
The rest, go through EACH frame, setting ONLY the image for THAT frame as a visible layer.
When all of your frames are set, you can set the time interval for each frame, from 1/10s, on up.
After interval is set, on the left side of the animation panel is a drop down (I believe its drop down), and you can select continuous loop, X loops, or Single Loop; this will set how many times the .gif will play through itself.
Once all this is done, go to File>export for web and devices.
MAKE SURE YOU SELECT GIF for you save type, NOT PNG, which is the default.
Congratulations, you have an animated .gif image.
I believe this is also the same method for CS3, but I could be wrong on that part.
