AirPlay mirroring will probably never be available. From what little is known, it does not simply take the image from your iPad screen and send it to the ATV2, but instead works by sending the apps themselves directly to the ATV2 and running them natively. This reduces latency and provides a smoother experience, but means it only works with Apple's processors.
To get mirroring to work on a PC would require an emulator to be written that runs iOS, in addition to reverse engineering the decryption that protects the whole system. Even if someone actually went through all of the trouble to do this... Apple could defeat it with a software update, rendering hours (if not months/years) of work pointless. Tongue If you want mirroring, buy an ATV2.