Based on a 1995 US patent, it seems Trend Micro are threatening to sue Barracuda Networks for the latter's inclusion of ClamAV in a firewall product.
I think Groklaw did a good article with info and links (thanks PJ and Sean): http://www.groklaw.net/article.php?story=20080125135544713
Other articles: http://yro.slashdot.org/yro/08/01/29/1313206.shtml http://arstechnica.com/news.ars/post/20080129-barracuda-defends-open-source-...
Related Wikipedia articles: http://en.wikipedia.org/wiki/Clam_AntiVirus http://en.wikipedia.org/wiki/Software_patents_and_free_software