I don't believe MPEG-LA will sue at all. They do not own the patents, individual pool members own the patents. If some members of MPEG-LA want to sue VP8 they might be out-of luck cause their specific patents are not infringed and they need to get other members on board for a stronger case in court. I doubt that *all* MPEG-LA members will ever agree to go after Google because most of them will be afraid of individual counter-suits which they might loose.
MPEG-LA patent pool was set up to prevent these companies from suing each other (which surely would end up in a huge industry wide mess) and some of the smaller members would like to use something else for free =) cause currently they are paying more to MPEG-LA that they are getting from it.