Here's my view:
M$ has benefitted from the head start, even if the Wii is beating it. The biggest thing is ... it launched before the PS3 and that is all that really matters - its direct competitor.
Here's the deal - the reason the 360 is still outselling the PS3 is because of folks waiting to see what it was going to do.
Folks held off buying a 360 in its first year because at the price points, several gamers couldn't afford to buy two $400+ consoles at once. So ... the *potential* 360 and PS3 customers waited to see which console would start to pull away from the other.
The 360's head start, as others have stated before me, offered devs a chance to learn the ropes on the 360 without worrying much about another console at the moment.
Now, the devs are hitting their stride with the 360 - and the install base reflects that. Had the PS3 came out at the same time, it would definitely be a different story. Folks would have leaned toward the PS3 because of the past successes. Sony would have kept the 20gb and 60gb with full BC - and had no need to make an 80gb. M$ would be backed into a corner, pretty much like the PS3 is in.
Now, devs are finding it's much, much easier to develop for the 360, the user base and attach rate is unreal and they are realizing the main reason for the PS3's user base being high is due to the way Sony's touting it - a Blu Ray player that plays games too. Think about it - why buy a standalone BR player when you can get a video game console that plays video games and BR movies for the same price, if not cheaper?
And folks are responding. The potential customers, PS2 owners, aren't as impressed with the high price point, lack of features AND dazzling gaming lineup of the PS3 and are turning to the Wii or 360. With the arcade price cut, folks are taking all this to heart ...
M$ has the clout to start demanding exclusives (or lack thereof). Sony will have to settle for scraps - nice, pretty games, but ones that won't move systems. And seriously, it would take an act of God for Sony to make a comeback ...
The only person who made a comeback like Sony's attempting was Lazarus ... and he was dead for just a few days ...