Dell is now selling a line of desktops dubbed the n-series that can be purchased with no OS installed. As you might expect, it does not save you as much money as you might hope.
I just did a quick configuration online of the no-OS system versus an identically configured unit with Windows Media Center on it using the C521 base. Upon first finishing both configurations, the no OS unit was actually $20 more. That was not a perfect comparison for one reason, though. The Media Center PC had a free upgrade to a 320 GB HDD, while it was $90 extra on the n series. Discounting that, the unit with no OS was $70 cheaper.
So is this a good deal? It all depends on the deals for individual systems/components offered at any given time, and your needs. If you just hate the idea of any of your money going to the big Microsoft machine, then that fact alone may be good enough for you to skip the OS and pick up an n-series.