While I don't know the patents, the MPEG-LA can only protect against the patents that are committed to that specific portfolio.
Even if Motorola are a member of the MPEG-LA, if they are suing for a patent that they have not committed to the MPEG-LA, then Microsoft have no protection.
About the worst that could happen is Motorola get kicked out of MPEG-LA.
I don't think getting kicked out of MPEG-LA is so easy. If Motorola is a member and does have patents committed to their pool, wouldn't a kick mean that those patents that are submitted would no longer be part of the package? If that's true, then Motorola would have even more bullets.
This only goes to show how nobody is protected from patents, even if you're careful about buying licenses.
Oh the irony. Wasn't the MPEG LA consortium supposed to guarantee protection against that kind of attack ?
Any details on the infringing patents ?