"Right now, Microsoft could continue to distribute under the terms of the V2 GPL..."
"This all depends on how much of the code MS distributes is actually relicensed exclusively under GPL v3, and no longer under GPL2. Otherwise MS can carry on with GPL2 distribution, and nothing changes"
Not true. The originator of the work can mandate that the work be distributed under any later version of the GPL. It explicitly says this in the GPL. Further more even if it didn't, contrary to popular belief, releasing code under GPL does not surrender copyright, hence the originator is free to withdraw everybody's (including microsofts) right to distribute under GPL2 then re-release the code under GPL 3.
See linux logitech driver fiasco for proof of ability to withdraw GPL license.