rcomeau, I understand what you are saying, but you are making an invalid assumption. You are assuming that the only way to make the iSight camera visible to a Parallels virtual machine is to connect the actual iSight hardware to the virtual machine - as is done for external USB devices.
However, there is no particular reason to do this. The other way to make hardware visible to the virtual machine is to emulate it. This involves pretending to be some generic piece of hardware and mapping accesses to that hardware to API calls to Mac OS X. This is how the drives, graphics, sound, network, CDROM, keyboard, mouse and most other hardware for Parallels is managed. To the virtual machine it appears as if there is an 20GB IDE drive in there. In reality, there is a file in the Mac filesystem. The difficulty in doing this lies in the implementation of the generic hardware interface not in the access to the real hardware - in the case of the harddrive, accessing the "real hardware" is done by normal filesystem reads and writes.
In the same way, the iSight camera can be accessed by standard Mac OS X API calls - any app can do it. Since one can access the video feed, one can then pretend to be a generic USB webcam presenting that same video feed. Then the standard generic webcam drivers in Windows can use the iSight.
Look in the devices section of your Windows virtual machine and you will see that nearly all of the hardware that Windows thinks it's seeing doesn't really exist. That is why you can install an out-of-the-box copy of Windows or Linux and it will work without any Apple or Parallels special drivers.
Supporting the iSight camera is entirely within Parallels abilities - in fact they could use the OS X APIs to support any Mac OS X supported webcam, not just the iSight. Whether they want to do this is another question.
(I am a professional programmer.)