This would be a first for the series, whose last outing - GT5 Prologue - was released in the US four months after it was out in Japan.
Speaking with Multiplayer, Yamauchi said that "its release is not too far away - it will be after the PSP GT release - and, most likely, it will be a simultaneous worldwide release".
If you can handle a bit of Italian (or Japanese!), the full interview is below.
Gran Turismo - Videointervista a Kazunori Yamauchi [Multiplayer, via 1UP and GTPlanet]