Personally, I'd love to upgrade my version of iDVD (It's at 3) by itself. However, I have to shell out $80 for five apps, three of which I've never launched. There's no way I could purchase iDVD separately, is there?
You use iDVD and not iMovie to? I can't think of a way to only upgrade iDVD. There is a lot of stand alone DVD burning software but they all cost the same as iLife so you might as well buy that.