Flamehead is wise.
It's my feeling that once Polyphony got a real good look at Forza 3 last fall, they decided their current best wasn't good enough to 'top' it. This was probably made more apparent after they got a retail copy and played it more extensively.
PD has always set the bar very high, even for themselves. Now, as Flamehead pointed out, they are pushing expectations even higher with another delay to the point where nothing less than a REMARKABLE game that's virtually flawless will be expected.