Well, if we look at 2008 we get a similar picture. PS3 riding on the heels of an incredible $100 price drop outsells the 360 (Though the margin is quite wider this time) for the holiday season and retains its momentum for the next 8 months with periodic system selling exclusives.
However as soon as Microsoft's pricings start taking hits around September and Sony fails to respond, the 360 begins selling to a more expansive market and starts getting some pretty good exclusives of it's own.
The biggest difference between the flip-flop of mementum between Q3 2008-Q32009 and Q32009-Q32010 is that Microsoft responded to Sony's price cut this year, as well as back in Q4 2007 when we had a similar surge following a price cut for PS3. It's worth noting that in 2008 when Microsoft stole mementum from Sony's machine that Sony never put up a fight. But back then they were never really in a position to with the PS3 being such a huge loss maker for the Division. However, with the new slim design and I assume a new 45nm RSX chip that should be coming soon for them, I'd say for once Sony is actually in a position where they can counter Microsoft's potential price drop this year, and if they do, they may be able to retain momentum from it shifting to Microsoft's console. However, it's just as possible that Sony will continue with history and simply focus on profitibility this year along with an agressive price drop next year.
Another factor to consider is the exclusive lineups and the motion controllers/cameras. Predictably Sony's lineup for the first 6 months of 2010 is a head above the 360's with big games launching in every region of the world while the 360 has some as well, but not as many, none as big as Sony's biggest, and none in Japan. However we we look beyond the first six months decidedly different positions surprise us. Microsoft has two very big set releases with Halo: Reach and Fable III, while Sony is a blank slate outside of The Last Guardian and potentially Agent as well as Final Fantasy Versus XIII in Japan. Really though both companies have a number of games left to announce for later this year so looking at things now too closely isn't probably too wise.
As far as Natal and Arc are concerned, it's also too early to tell which if either will be a hit. Arc has the advantage of being more assessible to developers and consumers already comfortable with the Wii's design to motion control as well as already has a market halfway established with all the PlayStation Eyes already in millions of comsumer's homes. Natal on the other hand has technological pontential and hype on it's side. However, I will say that Natal's advantages can be double-edged swords. Technological potential is great, but it's still just potential. If when the technology releases and it doesn't meet up with the expectations that people have placed upon it, then all that hype for it will turn against it and you won't be able to find a discussion about the thing without hearing the words "overhyped" or "dissappointed". If Microsoft does deliver on all of Natal's promises though, and follows it up with some convincing software for both casual and hardcore gamers, then it really does have the potential to be a big system seller.
TL;DR Bottomline though is that Microsoft still has a 5 million lead and doesn't have to do something until September and regardless whether they do something sooner this year or later, there is still a reasonable chance that Sony might steal momentum back immediately with another price cut this year. It's best for Microsoft to just wait it out and then make a dramatic rebound with a price cut, natal launch, and a slew of exclusive titles, and hope that it's enough so that even if Sony does counter, it won't be enough to keep the momentum from shifting.