Recently Microsoft started talking up a storm about it patents and how F/OSS infringes them. But as many are noting, it's just a ploy. Well...
For starters, perhaps this is what Ballmer has been working on for the last few years since he took over the lead from Gates. If so (and I have not way to confirm that), then that is quite interesting as it would reveal the difference between Gates and Ballmer and how they have lead Microsoft over the years, and who is possibly to blame for some of the mis-steps at Microsoft. But that's only speculation. On for the real stuff...
First - let's assume that software patents in the U.S. are valid. In which case, it is likely as others have said that they don't likely have much to stand on as a lot of it is likely basic stuff that more likely than not has a lot of prior art to it. Of course, they do have patents on NTFS and FAT; but those are not likely very enforceable since they have let them go so long - non-licensed FAT implementations that would be infringers have been available for more than 5 years, so either (a) the implementations provide prior art that would invalidate the patents, or (b) they would not be able to enforce them due to patent laws. - if you don't enforce your patent within 5 years of knowledge of infringement, then you lose the rights to the patent and enforcement. At least for FAT, time should be up; and I would find it very hard to believe that Microsoft did not know about the FAT implementations in Linux before 2002. Other patents that Microsoft has likely have a similar problem.
Now, of course, for the above, we assumed that software patents are in fact valid in the U.S. While the Federal District courts have held them to be, the US Supreme Court (SCOTUS) has not upheld that yet, and in fact SCOTUS has been actively working as of late to correct the mistakes of the Federal District Court - and Microsoft knows this. There were two rulings by SCOTUS recently that overturned positions of the Federal District Court that have been on the books for nearly 20 years, and Microsoft was the defendant in one of those rulings - AT&T vs. Microsoft. In fact, if you read the footnotes in the ruling from AT&T vs. Microsoft, it seems that SCOTUS is looking for a case to rule on software patents, and likely rule them as invalid. Microsoft would not want to be that case, but if they push the F/OSS community they will likely be backed into that corner - and patent threats will do just that.
So what is likely to happen - well, either Microsoft will wisen up and forget about it (not likely given their history), or they will push companies and the F/OSS community until their patents are revoked or software patents themselves are completely invalidated by SCOTUS.
In the meantime, don't believe their statements, and don't fall prey to their FUD.
Note: Edit - Corrected title from 'thread' to 'threat'.