For those who don't know: M'Vila has actually suffered a bit this season in terms of performances by (surprise) going forward a bit too much. In fairness, he is the king of Rennes and the team completely depend on him to act as their shield for the defense, and as the source of their attacks. They also want him to get goals too because Rennes are pretty shit going forward. He also never gets injured (though that'll probably changed) and started looking a bit heavy legged the second half of this season.
For whatever reason, the fans have turned on him because he hasn't been giving 9/10 performances week in, week out (he truly was a beast the season before), and it's obvious that he needed a change. As capi said, the early signing depends a lot on him but I still find this all a little too neat. Yann still has another 2 years I think on his contract, so Rennes do have leverage in keeping him until after the Euros. Unless he really didn't get a better offer from anyone, I find it strange that he signs for Arsenal so early, especially given the huge offer Inter Milan have (supposedly) tabled.
Like I said, if there's nothing news wise here I'll have to consider this probably a load of crap, but again no smoke without a fire, etc...