Typically the first generation of games released immediately after a video game console launch are graphically and technologically less impressive than games in later generations. A good example of this is the comparison between the original Halo (an Xbox launch title) and Halo 2 (which was released this past year).
Halo 2 is widely regarded as a game that pushes the original Xbox to the limits, whilst Halo looks aged in comparison. Despite the great increase in graphics and features when the two games are put side by side, developers haven't been given any new hardware to work with. They've merely had the time to be able to take advantage of all the features of the same console.
With this in mind it's not surprising to find that, according to The Inquirer, every single Xbox 360 launch title is single threaded. Apparently during a talk on multithreaded programming, a Microsoft speaker mentioned that all first generation Xbox 360 games will not take advantage of the Xbox 360's two other cores. This probably isn't worrying Microsoft too much, since they've given developers plenty of time to perfect their games before the PS3 and Revolution launch sometime next year.