Probably nothing.
I strongly believe that Microsoft was already trying to become a gatekeeper a decade ago setting the stage with Hailstorm. Only the fear for anti-trust authorities blocked them IMO.
Now Apple cleared the way and Microsoft already has a much stronger infrastructure in place to become a gatekeeper. I'm pretty sure they will fail at becoming THE gatekeeper, but I think they will still become A pretty large gatekeeper.
People should have voted with their wallet back when the walled garden xbox and iphone were released. But we are talking about the huge mass of consumers that simply don't give a damn.
Consumers swallow pretty much everything, just think about DRM and e-books. We were lucky enough that the industry got caught pants down by Apple with music but they learnt immediately from that.
Besides, both Microsoft and Apple were smart enough to let people think that, after all devices, can't really be locked down. The first xbox was a joke with a very convenient place to insert mod chips and the iphone was immediately jail-broken. But we all know this can be fixed by their side.
Unless things changed, the xbox 360 has very limited hacks to run game backups, but can't run unsigned code (it certainly can't run XBMC which was the reason why I bought the original version).
Apple TV, if rumors materialize in something real, will be the next big appliance following this path. Then, when times will be mature, jail-breaking (hacks in general) will magically become useless and that's it.
Also: Apple has been a little bit more evil by tricking a whole generation of developers (and developers wannabee) that they can make the ultimate app and become uber-rich while we know from numbers that for the vast majority of them income is ridiculous.
As I said, I'm not sure something can be done by us. Of course states should be there to prevent exactly this kind of behaviors but we all know how it works with politics and lobbies.