More powerfull intervalometer!
If you use your camera tethered to PC via USB, you can use the remote capture facilty of Canon ZoomBrowser. You can set the interval time in hours/secs (I've done great looking clouds streaming across sky type movies with it set to 15secs which you can't get in-camera). As you can download the pics straight to your PC, there seems to be no limit to number of pics (I've made movies from 500+ shots).
I use JPGVideo (freeware) to make the final AVI movie:
A bit more about this in another post of mine: