According to a patent filed by Microsoft, Microsoft wants users to make sure that their online avatars resemble their offline appearance. The patent implies that an individual cannot fake their online and offline apperances, no matter whether they are obese or thin. The abstract of the patent states:
An avatar generator for a virtual environment reflects a physiological characteristic of the user, injecting a degree of reality into the capabilities or appearance. Thereby, many of the incentives of the real world are replicated in a virtual environment. Physiological data that reflect a degree of health of the real person can be linked to rewards of capabilities of a gaming avatar, an amount of time budgeted to play, or a visible indication. Thereby, people are encouraged to exercise.
This makes a lot of sense for Microsoft’s project Natal – A Xbox 360 add-on which allows users to control and interact using gestures and actions, thereby eliminating the use of a controller. One of the possible uses mentioned on the patent page is:
a game competition could enforce that only requisite health levels are allowed to compete in a certain competition level. 
This might actually take a game like boxing on the Xbox 360, with Natal, to an entirely new and real level. Going through the patent, the main idea behind this seems to be bridging the gap between real and virtual gaming. Avatar anyone? The movie, I mean.