No spam flag on my system, but here are the SpamAssassin results from the header of that test message:
X-Spam-Status: No, score=3.374 tagged_above=2 required=3.55
HTML_MESSAGE=0.001, MIME_HTML_ONLY=1.457, RCVD_IN_SORBS_WEB=0.619]
The big ones (with links to descriptions):
There have been some SORBS blacklist changes recently, and even two weeks ago or so there was talk of them closing down. Perhaps related to your problems. Check your info on their site: http://www.us.sorbs.net/
EDIT: Additionally, since your test message was non-commercial, it was free of some extra scoring that might have otherwise hit it. Since you're so close to my cut-off level with just a plain message, a fuller message might have pushed you over the top for at least a flag. Only a few more points from there to total rejection.