I purchased a Canon remote interval timer off ebay. First copy was defective, a prompt replacement works fine. A co-worker was so impressed, he purchased one for his Pentax too. Here is the original thread:
Time laps photography / film: Pentax SLR Talk Forum: Digital Photography Review
I've been having fun making cloud dynamics animations. By shooting JPEG only at 2Mp and *** quality, I can get almost 4,000 frames on a 4Gb card. It is recommended to use an A/C adapter when shooting time-lapse, but I haven't had a problem with the single battery. When I set it up for a couple hour shoot, I mounted the grip and used both batteries.
If you purchase the ebay timer, make sure it has the correct pin-plug to fit the Pentax. There are two different Canon plugs.