I scanned through Google's Link Schemes page that they link to from their Webmaster Guidelines and found an interesting addition. It looks like the spam links that we frequently remove from forum posts are on the list for possible penalties.

Google may also be getting ready to target signature links with optimized keywords.

Link Schemes

Check out the section with examples of unnatural links.

Forum comments with optimized links in the post or signature, for example:
Thanks, that’s great info!
- Paul
paul’s pizza san diego pizza best pizza san diego
Think about all of the link spammers who have embedded unrelated links in forum posts on perhaps hundreds of forums where they have no ability to remove those links. We have been doing these clowns a favor by deleting their links.