Beginning of the end?
I don't think so. I'm sure Microsoft are working on a successor (the 360 has been on release for almost 6 years after all) but there is a huge Xbox 360 userbase out there, and they're not about to disappear anytime soon. The PS3 has done well but hasn't had the same degree of success as the Xbox, with part of the problem being the initial price at it's launch, which was crazy - and also the lack of an epic game - such as GT5 - for so long. The Xbox has kinda become synonymous with the hugely successful Call of Duty series, even though they are also available on numerous other formats, and this has helped it no end. I know that I spend about 90% of my time on Xbox Live playing COD, be it MW2 or Black Ops, and between that and Forza they were my main reasons for getting the console.
With regards to people having 'allegiance' to one kind of console - seriously, grow up.
I'm old enough to remember the insane bickering that went on back in the day between SNES and Megadrive (Genesis in USA I think?) owners . . . . we almost had fights on our bus going to school because of people arguing over which was better. Who cares?? As long as you enjoy your favourite console, there's no need to run down every other console or try and convince people that your opinion about the console is more valid than theirs.
I have an Xbox 360 slim which I love, but I also fully appreciate the PS3 and PC gaming experiences - and if I wasn't careful with my money, I'd probably end up with one of each console. Again, harking back to the old days, when I had a SNES and Gameboy, I also loved the odd time I'd get a play on a Gamegear, Saturn, or whatever. They all have their own pros and cons . . . . . . . variety is the spice of life people!