Just one word ...
- - Estoppel

More properly "Estoppel by acquiescence".
See Wikipedia for an explanation http://en.wikipedia.org/wiki/Estoppel

In this case, if Microsoft KNOWS that patents are being misused, they must assert their rights properly, or risk losing legal standing by effectively waiving their rights. Since Linux is publicly sourced, identifying the offending code and the specific patent would be trivial.

Not doing so, once they know infringement has occurred is legally binding.

Microsoft also has an obligation to "mitigate" damages due to a misuse of their protected property. In this case, mitigation takes the form of identifying the wrongful code and allowing developers to correct problems. Simply allowing protected ideas to continue being spread in Linux source effectively caps any damages they can claim.

Having asserted a claim of infringement, without documenting the claim, may foreclose any damages going forward. It is an effective waiver of rights.

Willful infringement -- if Linux continued to use Microsoft protected technology, that would be wrong, and grounds for a lawsuit. Accidental infringement, promptly corrected, is not properly grounds for legal action.

Any claims by Microsoft, without specific details are pure FUD.

A final issue, the patents Microsoft is claiming may not even be valid. The US Patent Office expects an applicant to do a thorough prior art search. However a cursory look at some MS patents reveals that they would not stand the tests of obviousness and prior art as written. Actually testing those patents in court might end up with the patents being invalidated.

Anyone seriously interested in pursuing this should visit the Groklaw website at http://www.groklaw.net/