I got a couple Wired links here that talk about this referral log spam, and the comment spam too.

The first one covers comment spam. Their theory is that the spam comments aren't to generate web hits, but instead to boost their site's Goole Page Rank.

The other link talks about the referral log. Some sites I have seen (specifically Moveable Type blogs), display their most recent referrers. By inserting spam into the referral log, the guys can once again boost their Google Page Rank.

Interesting.