My wild guess is now that Sony announced the PStwo (according to rumors), they might actually be holding off release of GT4 to accompany the release of the new PS3 rumored for next spring. After all, what was the single biggest game to accompany the release of the PS2 that pretty much guaranteed every young, hot-blooded male in Japan would rush to get one?
It wouldn't be surprising to hear that the DP developers got tapped on the shoulder and told that they were given an extended deadline to polish up the game except that it would only be for PS3... Sony's already sold a bajillion PS2's so it's not like they need another game when they could definitely use it to hype the new platform.
Of course, hearing from a buddy that HIS friend working on the OS localization team of PS2 in Tokyo was laid off recently... not only himself, but the entire group!
On the other hand, all this paranoia could be from playing way too much Prologue; I'd almost be tempted to sell off my PS2 (Japanese) and copies of GT3, GT3 Motor Show, and GT4 Prologue...