end of 2011 is my guess
keep in mind the sony will keep support for the ps3 going much longer than microsoft will for the 360. they still support the ps2 to this day and it will be no different with the ps3
I'm sure a year after the new xbox comes out microsoft will probably not even make 360's anymore and drop it all together like they did the first box
also keep in mind that the ps3 is the best blu ray player in the market. in a year or 2 when the price drops to $200 and blu ray and HD are the standard in most houses expect to see many more consoles sold