The Qt4 backend appears to be broken in the Mac py2.6 binaries, using OS X 10.5.7 and the latest version of the Qt SDK from qt.nokia.com. I don't have the machine handy right this second but using plot() from an IPython interpreter with my backend set to Qt4Agg causes a hard crash for me every single time.
I should note that TraitsBackendQt seems to work fine, so at least PyQt doesn't (on the surface) appear to be at the root of it.
I'm wondering if it's a peculiarity of my system. Can anyone reproduce this? (this is an Intel Mac, by the way)