I stumbled across
this website recently. The guy talks about how he does it, originally with film and moving over to digital.
It's something I've been considering doing, my thoughts are that the simplest way to do this would be to buy a cheap Canon point and shoot, and put
CHDK on it, as you can use it for time lapses. It's got a list of supported camera so it's just a case of finding a cheap one on ebay. If it's any help, I rustled up a quick page to search ebay for all of the cameras on the list which should be compatible and display them in one window -
http://whoracle.com/images/ebayframes.html (only the canon ones are compatible, some other manufacturers show up with the same model name, ignore those)
It'd be necessary to power the camera from a different power source, the guy I linked to before seems quite fond of using a motorbike battery, I haven't looked at voltages but you'll probably need to lower the voltage, in which case a buck (or buck-boost) converter should do it. You could also use a voltage regulator but depending on the voltages would possibly waste more power.