As a few have said the next 360 would have to be programatically compatible and backwards compatible with the current 360 to come out that early...
That way developers could develop 360 games that worked on all 360 versions or develop for a new version that probably was Multi-DVD (HD, Blu-Ray, DVD9) based, and had a wilder video GPU and extra processing power for games that are coming on the PC (Ex. Crysis).
But I think it will be based more on market conditions then on a cycle that Microsoft wants... If 360 is going strong, they may opt to delay and put another year into R&D, but if PS3 is winning then such a move might work if executed properly.







