I'm not saying they will or they won't, but there is no point wasting resources assembling a patent pool against VP8 if it isn't even popular yet. When it becomes a threat, you can bet then the MPEG LA will assemble something, valid or not.
And the thing is, hardware H.264 will almost certainly be in every phone built until the patents expire, so ragging on a free alternative is just going to jeopardize their existing revenues, for a shot at the hacker community's websites like Wikimedia running H.264 and paying royalties - which will never happen anyway. Meanwhile most likely future for WebM is that some forward thinking companies serve WebM if you're using Firefox/Opera, but most are lazy, pay their H.264 tax, and require you to use IE/Safari/Chrome.
You can also compare ffvp56 and ffvp8, although there's a larger time gap between the two implementations.