Simon writes that open source has some trouble with unwitting infringement of patents, and (following the thread and replies in the comments), that while OSS is great at demonstrating prior art, it seems to fail at the usual commercial tactic of maintaining thickets of patents to cross-license when one needs someone else's patent.
This seems as good a time as any to wonder whether software patents make any sense. I mean, the whole notion of patents stems from an assumption that inventing is hard and expensive. This is true if one is inventing in physical or chemical processes, where the equipment needed to perform research is beyond the means of all but the well endowed.
It is less true in software. We used to joke that a good software engineer discards half a dozen good inventions before lunch each day, and the equipment requirements to invent in this space are essentially zero (at least for those in wealthier nations).
So, do patents actually serve us? Since they don't make up for any particular problem in development of software inventions, they tend to over-reward early participants in the field for writing down the obvious. I mean, the XOR cursor? Or the wireless modem? (Despite the notion of a phone modem and the notion of a wireless phone both being established, those bright minds at the patent office actually granted a patent on the trivially obvious combination!)
To my mind, patents in software add no value, and really only serve to subject software development to the less than scrupulous (lawyers, that is).
And the open source world seems tailor made to run into difficulty here, since (as others have noted) there is no self-policing (can't be because by and large developers have no clue what patents have been granted), no legal staff to produce defensive patents and maintain them for cross licensing, and limited resources to fight a legal battle with a large patent owner who might choose to litigate (prior art may be enough to have a case, but one needs legal resources to successfully defend).
It is starting to look to me as if the OSS world won't ever meet its real potential unless software patents go away. Since the non-OSS world has little incentive to change, it would seem to be up to OSS boosters to drive it.