tedster - 9:50 pm on Feb 12, 2011 (gmt 0)
As a forum administrator, I'm certain that forum profile links are far from all spam. There's important data in forum profiles around the web that Google wold be foolish to ignore.
I think the link weight issue is much more complex than even we can guess - and if we saw all the data that Google engineers have, we would all be boggled. The scale is immense, as is the data churn.