Recently, federal courts have reined in the counterpart "copyright trolls" like Righthaven LLC, who purchased copyrights to (usually) news articles and sued the nonprofits and organizers of internet message boards for settlements that often drained the coffers of these organizations. It was recently found that several of Righthaven's cases were wrongly decided, and they should return the monies extorted, but of course they are already bankrupt.
With patents, it's slightly trickier. A non-practicing entity (the polite term for patent troll) may be non-practicing as to a particular patent, but it could still be in the R&D phase (think clinical trials for pharmaceuticals) or a patent on a product that is not sold because it's proprietary in-house software (Skype's encryption software). This article gives a solid outline of the legislation that has been taking shape to combat patent trolling and makes some very interesting suggestions.
Remember, if a patent troll (or anyone) sends you an angry letter, don't just send a check! Always have legal counsel reveiw both the entity and the underlying validity of the claims before making settlement offers. Some of these may be legitimate patent owners with a grievance or good-faith beleif you infringe, and some of them could be as