As a follow-up, for those interested, this appears to be WordPress profiles. I recently added Q&A to my site, which meant I had to turn on Wordpress registrations. I had upwards of 600 subscribers in the span of a few months. I'm guessing a large portion of them had spam URL's. I deleted all 600 and submitted a reconsideration request. I'm also removing members from my XenUtils sitemap submission. Added this XF plugin to allow links after a prescribed number of posts.
I use XenUtils spam finder daily as well.
If this could happen to a website because of WordPress profiles, I can imagine it can also happen to forums. While I'm not 100% sure it's the cause of the warning, I'm betting it is the problem. Still, this could be writing on the wall for forum admins. You have to be vigilant against spam or you'll get hit with a similar note from Google. Of course, if you own a forum, you absolutely must have a Google Webmaster account.
Still in the index. Hoping to stick and for a quick resolution. Scariest thing ever.