It is spam. Debated heavily on vb.com a while ago.
Basically they register and fill in the homepage link. If the profile pages are public or if the homepage link appears on the memberlist, then they go up the Google PageRank.
Best defense:
1) Take homepage off of your memberlist.
2) Make member profile pages members only (no guest view).
Just take a look at some of the URL's set as their home pages