My friend Hoeken has been working on this and putting together a nice software system for printing stuff out with the reprap.
The host software on the computer takes STL files and slices them up and then makes machine paths for them.
Then it sends those paths in chunks to the arduino.
The arduino takes those commands and translates them into step and directions and that kind of thing.
If you want to work on that, Hoeken has done a lot of the work already and if you're into it he'll give you subversion access.
The stuff he's got going is at 
This is the part of the project that I am really worried about because it's out of my league. Help would be awesome!