Ironically, Sony originally intended to launch the PS3 during this timeframe, but later decided to move up their plans (not at the last minute or anything) because of the Wii/360 and HD format war. At least that's the talk.
Eitherway we've heard that Sony was still working on finalizing the some tools for third parties and that they wouldn't have delivered them in time to third parties for them to make use of them come launch. Pushing it back is Sony probably wanting to do this 'right', instead of having home be this huge W.I.P. come launch.