Most probably Microsoft. Considering that the only wheel out there for the 360 currently is the Microsoft one. You can almost assume that Microsoft did not give out any information as to how to make a wheel work for the 360.
Another thing to keep in mind is that the console (360) needs drivers to support the wheel too.
Now you might say "But Microsoft already has their wheel out, which means wheels are supported." However, knowing Microsoft, they might have done some shady tricks to prevent 3rd party wheels from working.
Hypothetically speaking, a 360 wheel might need to send some sort of data to the 360 in order for the 360 to recognize it as a wheel device and without knowing that data, the wheel will not be recognized.
I hope you had your tin foil hat on while you read that.
