While looking up such a device (expecting to find one built into some remote shutter release or cable of some sort), I found this. Cool. I already have one of those for my AP Calculus class, so that unlocks some time-lapse possibilities without spending any money.
For some reason, I continued searching for things like this, when I stumbled across a site called CHDK, which stands for Canon Hack Development Kit. It's basically a large database full of open-source coding that has been done to add extra cool features to your camera. It's kind of a hassle to install (and took me a while to figure out; it's kind of tech-y), but among the scripts, I found an intervalometer that works with my model of camera and my version of firmware. Looks like I get to do some better time-lapses before my XSi gets here after all.
I tested it out today. The setting was one photo every 45 seconds, stopping after 200 photos. I set everything up, and left it to its work. After a few hours, I came back and checked on the progress. The battery was dead. But it had made it to 177 photos first! The reason the battery died is because for some reason, the camera will not operate in manual focus with the LCD display turned off. So I am forced to leave the LCD on in fear of the autofocus getting off at some point during the shooting. Fortunately, the XSi is exactly the opposite of that: the autofocus won't work with the LCD on Live View.
Enough talking for now. Here's the video:
My apologies for the obnoxious color cast shifts. Of all the things to forget to set manually, white-balance wasn't the worst one. But it's still kind of a noobish mistake.
On the bright side, I've been hit with another realization: the choppiness of my previous time-lapses was primarily due to the human factor involved in the intervals between exposures. When the computer handles the intervals, it gets the exactly the same. No matter how hard I try, I'll always have a bit (or more) of error in my timing shots. So never mind worrying too much about finding a 1-frame-per-photo video editor, this already looks significantly smoother.
Bottom line: Yes, the XSi will do a better job. But it's cool to know that you don't have to have pimped-out equipment to do really good time-lapse photography.