I can sign up on a dozen forums, all of them running mybb, smf, IPB and VB forum software, add a link to my site in my signature and make several post. Go back to google 6 - 8 weeks later and type in the link:site address command - and most of the links are from forums running vbulletin. I noticed this a few weeks ago. There are around 6 - 8 MYBB forums I am a member of. Not a single site shows up in the google link: command. In my google webmaster tools, forums with equal number of post - vbulletin forums will register more backlinks then anything else. Has anyone else noticed this?
If a site is using whatever forum script but have a high PR and well-on-site-SEO'd then you're there. Also link:address doesn't really show everything.