EDIT October 9, 2008: Updated script to work in GIMP 2.6 (the newest stable version.) If you have a previous version of this script, please delete it to avoid conflicts.
Let me know if you have any trouble with it.
This is something I've wanted to do for a LONG time and now that I understand scripting a bit more, was finally able to get this finished! I've taken the default GIMP Spinning Globe Script and modified it to map a single layer onto a rotating cube. I've added the option to set the speed and adjust the size of the cube, but the default cube size is pretty good. You can't go much higher in size than the default because if I allow you to make it too much larger, it won't fit in the current window size (I *could* implement an option to resize the frame to allow for a larger cube, but that's not planned for the time being.)
Click on the Download link to open the script.
Save it to your GIMP scripts folder (ie....C:\Program Files\GIMP-2.X\share\gimp\2.0\scripts) and then go to Filters > Script-Fu > Refresh Scripts and it will appear in the main image window under Filters Menu > Animation > Spinning Cube... (Thanks to circledancer at gimptalk.com for testing this out in GIMP 2.2. It works fine, with no modifications.)
The script defaults to setting the x-angle to 30 degrees so the cube is tilted slightly towards the viewer. Later, I will add an option to allow the user to change this setting....easy enough to do, but I was just being lazy! NOTE: Depending on your computer, this script may take awhile to run. My home computer generated the animation with 10 frames in about 5 minutes....but it took about 20 minutes on another machine that is much newer and has more memory. So, I recommend you try this out with just a few frames at first to see how your machine responds. Maybe this is why the script was never written before!
Allow for "predictable", simultaneous rotation on all 3 axis. Right now, I can't figure this out.
Allow for mapping different images to each face.
Let me know how it works for you or if you have any problems with it. Suggestions for improvement are always welcome!