The 360 launched Nov 22, 2005 and the PS3 launched Nov 17, 2006 in North America. So that's a hair shy of a ONE year lead, not almost 3 like a previous poster said. That said... a one year head start is still a big deal. So, here's my breakdown:
PS3 - The PS3 is more powerful, period, we're not going to have that argument. It's doing well this holiday season and that pretty much means it's here to stay. Blu-ray is a wonderful format, as is HD-DVD, but with the drive in the system it has real and tangible backing unlike HD-DVD. The ability to have all regions on a single disc and the ability to support massive data storage on a single disc are definitely going to help in the long run. All models of the PS3 have a hard drive which means developers can, are and will continue to use it to cache data. This means faster loading games that eventually will run smoother too. The PlayStation Network is in its infancy and is progressing well. Sony is an axcellent student and is learning a lot from X-Box Live. Yes, the PSN has a ways to go, but it is evolving constantly.
XBox 360 - The 360 is a far superior system to the original Xbox in almost every way. XBox Live is better than ever, as was mentioned, the online experience on the 360 just isn't matched. The 360 has a year head start and has taken full advantage of this. Being the first Next Gen system it has sold well and built an impressive library that continues to drive sales. The system's pricing is better as well. The real issue is that the 360 launched a year earlier than the PS3 and isn't as powerful. Without a built in high capacity disc drive multiple discs and multiple regioned versions are going to become an issue over the remaining life of the system. Also, Microsoft simply dropped the ball by not including a hard drive in/with every system. The Core and Arcade bundles are great for comsumers looking to get into the Next Gen more cost effectively, but are going to seriously hurt them in the future because developers won't use it to cache data since they can't count on everyone that would buy the game owning a 360 with a hard drive.
Conclusion - The 360 will continue to do well for another 2-3 years and most likely maintain the lead it has over the PS3 though that lead may shrink in the coming years as the PS3 becoms more affordable and games being to equal or surpass the 360 visually. Sometime around 2010 or 2011 the 360 will start showing it's age. Developers will have all of their tools running much better for the PS3 and as many developers have stated they will start with PS3 development since it is harder and then port down to the 360. At that point Microsoft will have to launch a new console. This is where the issue really, truly lies. Microsoft will launch a new console as the PS3 is still gaining steam. They will hit a brick wall of consumers not willing to purchase another new console considering they already have atleast one and that many of them may already have Wii's or even Wii 2's and the new affordable PS3.
That's how I feel anyway. I own both. The 360 for gaming goodness now, the PS3 for the future.