Just read in LinuxDevCenter.com: Freedom, Innovation, and Convenience: The RMS Interview

FB: Do you think it's a good idea to port a free software project to a proprietary OS such as Windows?

RMS: Porting free applications to nonfree operating systems is often useful. This allows users of those operating systems to try out using a few free programs and see that they can be good to use, that free software won't bite them. This can help people overcome worries about trying a free operating system such as GNU/Linux. Many users really do follow this path.

However, we need to be careful to avoid suggesting that the purpose of free applications is to be used on a proprietary system in that way. Using free applications is a step forward, but it doesn't take you all the way to freedom. To reach that destination, we need to avoid the proprietary software that denies users their freedom.