I was referred to this on the EXRX forums. It's a short article on getting your first pullup.
I think it's got some good suggestions, especially the seated pullup. I don't think curls are worth the effort, though - it's terribly unlikely, in my opinion, that you are being held back from your first pullup because of weak biceps. I have nothing against curls, I just don't think they provide enough bang for the buck when you are aiming for your first chinup.