Also known as the Go Pro camera:
GoPro - Wearable Digital Cameras for Sports

Several friends use them on pro drag cars, rally cars, etc... with great success.

I was more thinking of something that will modify the factory unit to record video from the factory cameras and store the footage on the factory internal iDrive hard drive. Basically something similar to in-motion video hacks. Perhaps a high quality aftermarket firmware update could achieve this.
If people can hack/jail break an iPhone, I'm sure they do it to the iDrive.
Would I do it? No

__________________
'98 Type R drag car ~1400hp
'01 X5 3.0 silver (sold)
'02 X5 4.6is silver (sold)
'04 X5 4.8is black (sold)
'10 X5 ///M Alpine White