Tone is correctly identifying one potential source of the problem. What turned me onto the induction box was that Cyrk mentions the smell during vent (fresh air) operation. Try cleaning the outside first as it's easier and cheaper, then if no improvement move to interior side of the system. Funk in the outside compartment and corresponding fresh-air plenum is much more common.

This is a common product for treating outside intake: Ultrashield.

Some mechanics will also suggest that you can help the situation by turning off your air-conditioner but leave the fan on for a few minutes before you turn the car off. This will help dry out the moisture in the evaporator that can cause the smell.

Some more tips on A/C odor: AutoTips Fresh Air Tips.

A good ole Yahoo/Google search will yield some useful tips from others with the same or similar problem as well, such as on this BMW forum: "Dirty Sock" Syndrome.