From http://www.linuxjournal.com/node/1000111 with my comments interspersed:

Richard Stallman

"Now, what we didn't have 15 years ago was the threat of making the program effectively non free by technical restrictions placed around it. That's what Tivoisation is. Tivoisation means taking a free program and distributing a binary of it, and also providing the source, because the GPL requires that. But when the user changes the source code and compiles it and then tries to install the changed program he discovers that that's impossible because the machine is designed not to let him."