I love this guys work. I had a similar idea of doing an HD clip of the trees changing color this fall with my 20d. I think I have a way to pull it off without having to keep my camera in the middle of the trees for 2 months. The problem is getting up early enough to do it before work so I don't think I'll make it this year.
This guy did this using the Canon TC80N3 remote. I'm not sure if the same remote will work on the 350d.
I read on a forum that he used a 2 second exposure and took the pics one after the other. Once the shutter closed he took another one. He then used Motion to assemble to pics together. Oh yea, he said he stayed in manual exposure mode.
I'd give you the link to his forum post but I didn't bookmark it. Search
digg / All for his video and in the comments there's a link to it.