The MPEG-LA is not new to empty threats - the company, led by Larry Horn, a known patent troll, has been making claims about Theora for almost a decade, but never actually backed up said claims. This was probably because as an open source standard with relatively little adoption, no money was to be had from launching a patent lawsuit.
With Google purchasing and releasing VP8 as open source, including a non-revocable unlimited patent license, there's suddenly money to be had from On2's technologies. Larry Horn, vulture that he is, is now smelling money, and as such, has started the process of forming a patent pool around VP8. There's no way of proving it, of course, but it wouldn't surprise me if Horn has made a few trips to Cupertino and Redmond as well.
This process has now yielded results, but it appears the process is far, far from complete. Horn claims 12 companies have stepped up claiming they own patents essential to VP8, but in true FUD-fashion, Horn refuses to name the patents or companies in question - thus preventing the web from investigating this whole thing.
This reluctance indicates Horn isn't particularly certain about the strengths of these twelve companies' patents, since if he was, he'd be shouting it off the rooftops. It could also mean that these companies are uncertain about their chances against fighting Google in court, and thus, want to keep the patents secret so they may patent troll the heck out of a smaller company implementing VP8. It's also interesting that it took the MPEG-LA four months after the deadline before they announced this news.
Google has already responded to this news, but it's a fairly default response. "MPEG LA has alluded to a VP8 pool since WebM launched - this is nothing new," a Google spokesperson told streamingmedia.com, "The web succeeds with open, community-developed innovation, and the WebM Project brings the same principles to web video. The vast majority of the industry supports free and open development, which is why we formed the WebM CCL enabling member organizations to license patents they may have that are essential to WebM technologies to other members of the CCL. We are firmly committed to the project and establishing an open codec for HTML5 video."
The WebM CCL already contains some industry heavyweights (like, but not limited to, AMD, Samsung, Texas Instruments), and the list of companies supporting WebM is immense, and includes just about every chip maker except Intel. This is a formidable group to take on in any legal fight.