Seems that over 1/5th of Windows installations that have used WGA have failed:

More than a fifth (22 per cent) of Windows installations failed tests on their authenticity, according to figures from tests conducted using Microsoft's controversial Windows Genuine Advantage (WGA) tool.

The BSA (Business Software Alliance) has chimed in predictably:

Figures from the WGA audit are lower than those from the Business Software Alliance, which reckons 35 per cent of business software is counterfeit.

Left unasked by the author of the story, or by MS, or by the BSA: How many false positives live in that 1/5th? I'm sure that many of the checks are from systems that have not been installed in accordance with MS licensing - but the real question is, how many have been, and failed anyway?