There are some great new technologies emerging for the Linux desktop, I think that as time goes on we will not only see the usage of Linux as a desktop increasing, but I think it will threaten Windows and OS X, plus the technologies will most likely be much better implemented. After all OS X is based on Unix and Vista leans on heavily on it too.
Take a look at this jaw dropping video showing just what is possible through the use of Beryl and XGL for the Linux desktop.
Ubuntu seems to be shaping up as the go-to distro for running Linux on the desktop. This is a great link outlining how to create screencasts on your Ubuntu desktop using OS tools.