Several people who read A Plan for Spam have asked if I'm worried that Microsoft has already been granted a patent on some aspects of Bayesian spam filtering. I'm not. A patent doesn't mean much until it is tested in court. Especially for something like software, where the patent office regularly grants patents for ideas that are not new at all. Jason Rennie's ifile, a Bayesian mail classifier, predates Microsoft's patent application by two years. Pantel and Lin's paper about using a Bayesian classifier specifically to filter spam also predates the application by three months. The one novel idea I see in the patent is using non-word features of the message (e.g. the arrival time) as if they were words in a Bayesian calculation of spam probability. But (a) this is an obvious idea to one skilled in the art, and (b) you don't need to do this to make an effective Bayesian filter. Even if the patent were valid, I don't think it would be dangerous, because I think big companies apply for patents mostly as a defensive measure. Big companies apply for patents on everything that comes out of their research departments as a matter of course, more to protect themselves against patent suits than to use as a weapon against competitors. Like many big companies, Microsoft wins by dominating distribution channels, not by having better products. Having a technical edge over competitors is not critical to their business. Patents are even less of a worry for free software. Even Microsoft is constrained by public opinion. Can you imagine the stink it would raise if Microsoft tried to shut down an open-source project for patent infringement? I've never heard of any company, big or small, trying to shut down an open-source project over a patent. However, if you're worried about ideas being taken out of circulation by being patented, the thing to do is publish every idea you have as soon as you have it. No one can patent an idea that has already been published by someone else. And if you want to start a startup and are worried about getting caught in a web of patents, build a server-based application. That kind of project is far too messy and hands-on for anyone to get very far into it in a corporate R&D department.