Whilst I don't enjoy them, I am wondering why MS are sitting on this IP.

I know they have Forza every other 2 years but a PGR inbetween would still do well I reckon.

Maybe they don't want it to compete with Forza but you are right, they could alternate between the 2 e.g 2011 Forza 4 2012 PGR etc etc, it could work.

yes

It's definitely time for a new pgr, it's the game I bought the original xbox for. And revive Rallisport challenge too please.

I've been looking forward to another PGR for years now, but, I doubt it'll happen. Bizarre Creations, the guys that brought us the original Metropolis Street Racer thru PGR 4, closed earlier this year.
Their last game, Blur, which they sank a few years into developing, barely made a mark on the sales charts and Activision closed them down.
Microsoft owns the IP for PGR and if they ever decide to use it, it will be a completely new development team working on the game, bringing a different vision and their own ideas to the PGR formula. It would't be the same game... at all.
Too bad about Blur, still not sure why it didn't sell better, it was a fun racing game.

They are focusing on Forza, and maybe they still haven't found the right developers.

I agree with those who say that they should alternate the two franchises (like EA with Medal of Honor and Battlefield).

There was rumor about PGR5 a while ago. Maybe we will see a new release come 2012?

I LOVED PGR on the original Xbox. IMO its SO much better than Forza and GT since its actually fun. Its the type of game that would make me envy Xbox owners so much that I would have to end up buying the console.

PGR2 on Xbox 1 was amazing. Maybe a 720 launch game

I remember hearing the same thing. Supposedly Turn 10 is working on it.