My friend Abhijit Menon-Sen writes:

The [Canon EOS d30 and 40D] cameras (and, as far as I know, all other EOS cameras) share one bad habit that has always annoyed me. In aperture-priority autoexposure mode, the wheel behind the shutter button controls the aperture and the dial on the back controls exposure (via a ±2 stop compensation). In manual exposure mode, these functions are gratuitously reversed, and the front wheel controls exposure directly.

It's clearer to think of it this way:

• In manual mode, the rear dial controls aperture, and the front wheel controls shutter speed.
• In autoexposure modes, the rear dial controls exposure compensation, and the front wheel controls the prioritized parameter; that is,
  • In shutter-priority, the wheel controls shutter speed.
  • In aperture-priority, the wheel controls aperture.
  • In program, the wheel "swaps" shutter speed for aperture, changing each so that the exposure is maintained.