Check out Whiternoise over at Hexus.net who has this great post about building a hardware timelapse device using programable chips. He has posted circuit diagrams and great shots of the plastic breadboard set-up.
Looks like a real fun project if you like that type of DIY – the electronics with wires and solder type – and if your Nikon DSLR is one with an IR receiver for a remote; e.g. works with a ML-3.