Your patent FUD: any software can be potentially sued for patent infringement. You can't prove the opposite. See Oracle vs. Google on Java patents and a number of other examples.
Pointing that out about WebM software specifically is not insightful. You can make the exact "but can you prove it doesn't infringe patents" statements about any software product.
There are, however, reasons to believe that WebM does indeed avoids patents.
1. It's a technology that has been created and commercially licensed by On2 for years. In all those years no one sued On2.
2. On2 licensees (including Adobe) were confident enough about the technology being patent-free to pay good money for the license.
3. Google, presumably, is not stupid and didn't pay hundreds of millions of dollars without doing a technology evaluation, including potential patent issues.
4. Even if someone sues WebM, Google has a better chance than most to defend it.
So while no one can ever satisfy people demanding impossible proof of WebM being safe from patent threats, there are very good reasons to believe it is indeed safe.