The folks over at the Software Freedom Law Center have gone ahead and estimated that Microsoft pays $21.50 per copy of Windows XP sold to third party patent holders, including Sun and Novell.

"If you run a computer using Windows, you are not just paying for the programmers who put the program together and the corporate operations that brought it to market," the group stated. "You are also paying a hidden tax of well over $20 that Microsoft has had to pay to other patent holders." The SFLC pointed out that Linux, by comparison, has never been found guilty of any patent infringements "making Linux a patent-tax-free alternative to Windows".

Perhaps the SFLC hasn't heard of Novell's SUSE distribution, since they are paying a patent tax to Microsoft for the distribution of "open source software shipped under the agreement" which may include Linux (at least if you take Steve Ballmer out of context).

But, this question really cuts to the raison d’être for this site: what are the specifics of the Microvell patent covenant? What open source software has Novell agreed to pay royalties to Microsoft on, for how much, and why? What is Novell's "patent tax" on SUSE?