Everyone was aware that it would be possible to run Windows on the new intel-based macs, unless Apple deliberately stopped them. Loads of people thought that they would stop windows being installed. But they haven't!
Apple has confirmed that they have not added any security which means that anyone who wants to can install windows on a Mac. This is highly unexpected, and potentially could increase take-up of Macs drastically.
The question is, will it be possible to install Mac OS X on a PC, now that they both run on the same architecture. I guess this is very unlikely, as Microsoft would not appreciate that as an idea, and Mac rely on Microsoft's goodwill to produce products for Mac such as Office, without which there would be very limited take-up of Macs because of compatibility issues.