I've been toying around with Ubuntu linux, seeing if I could make the move over to that side of things full time. It's gotten so much better over the last couple of years, that it's finally a viable fulltime desktop environment. The Ubuntu distro has an almost perfect balance of ease-of-use and hardcore-geek-utility. apt-get is great (but hardly user friendly).
Brian's been working on making the switch from MacOSX to Ubuntu (or UbuntuStudio), so I've been thinking about it again.
Most of the apps I live in are there (Firefox is a good enough browser - even if it isn't Safari, Thunderbird is a good enough mail app - even if it isn't Mail.app). It's got all of the server stuff I use either pre-installed or a simple apt-get away.
The one really killer app that is keeping me on MacOSX is Aperture. Nothing comes close on the Linux side of things. Nothing.
There are also tons of niceties in general use in MacOSX. Too many to list. And I'd miss them if I switched to Ubuntu.
Of course, with Parallels, it doesn't have to be an either-or kind of thing. I can run Ubuntu inside MacOSX on my MacBook Pro. But, if I'm already running MacOSX, Linux is a bit redundant...