Over time I find myself glued to Linux more and more: the software I like tends to be native on Linux and somewhat foreign on OS X, and yes - I know about Macports and I don't like it: too limited and too awkward compared to apt. All in all, OS X just isn't particularly hacker friendly: it always took me longer to do something on a Mac, the recent case was Python/Pylons stack with some libs: I was done under a minute on Ubuntu while Mac took some googling and IRC'ing to accomplish the same task.
Moreover, I keep hitting OSX limitations on less exotic stuff all the time: the terminal is retarded by default, Finder doesn't really do FTP properly (in 2008! WTF??), windows are resizable only by bottom-right corner, font rendering is worse than Ubuntu is capable of, Mac "spaces" are dumbed down version of Gnome workspaces - the list goes on and on. The only area where OSX destroys competition is their window management: I find myself Alt-tabbing a lot less on the Mac.
Also OSX is too commercialized: people are selling freaking screensavers for god's sake... So I am on Ubuntu most of the time now, the only thing that I am envious of is nearly instant ruspend/resume on my macbook and, as mentioned above, Macs window management.
But hardware plays an important role too. MBP+OSX wins head down against an average disposable Dell/HP/Whatever, even Ubuntu powers can't help that junk, so Thinkpads rule.