I've wanted something like this for ages! I have a small internal drive with my whole iTunes library on it, but I'd much rather have a large external drive with my iTunes library on it and a few select playlists copied onto my small internal drive.
I can't program anything more then some crappy PHP, but this is something I've wanted for ages. I'd even pay a bit of $$ for it- $10 is reasonable IMO, but free is best.
Actually, I've been wondering why iTunes hasn't been able to do this yet. I think with music libraries reaching into the 60+ gig range, and hard drives coming as small as 80 gigs, it's certainly something that a lot of people would probably find useful.