SizeUp requires X11, which is not included in Mountain Lion by default, but can be installed.

So I removed SizeUp from my startup items and didn't install X. I never use SizeUp (or, intentionally, Cinch), and I never run X, having years ago carefully removed the alias that would launch it. Why the comment? Old age. Being able to download and install X on Solaris, and then to be able to transcend the Unix command line was heady stuff. Each new release was a major download (megabytes!) of intelligently written C code. Bob Schleifer at MIT's Project Athena led the way, and it was heady stuff to be able to take part from the periphery. Wikipedia says it was launched in 1984, so I guess that's right. Still have a stack of old programming manuals for the system in the corner of my office. Maybe I should give up on OS X and go back to Linux where X is alive and well ... if not quite ready for the desktop yet.

Update: (March 2013) There’s a new version of SizeUp available that doesn’t require X11. So now I've got the best of both worlds: SizeUp and old-timer memories of X11.