I'm not so sure Python 3.0 was really designed to carry any real momentum for "Python 3000" instead, it seems to serve as a base platform for future migration. I understand Guido and the Python team haven't explicitly stated that, but that certainly seems to be what they signaled. I don't expect see widespread support until 3.2, with Python 3.1 likely being the signal for people to begin serious migration efforts. As a Python developer myself, I couldn't move to 3.0 even if I wanted to: none of the open-source libraries I depend on have even begun to look into 3.0 support (Numpy won't even support Python 2.6 until its upcoming 1.3 release).
Besides that, from a practical standpoint I'd guess the Unladen-Swallow team wants a codebase on more solid footing, like the mature 2.6.x series, as opposed to Python 3.x which is in a greater state of flux.
That said, I'd like to take a moment to express my frustration with installing or compiling PyGtkGLExt ( http://gtkglext.sourceforge.net/ ) on Windows with Python 2.6. The only installers I've found for PyGtkGLExt are for Python 2.4 ( http://www.stephanebrunet.net/gtkglext/pygtkglext-1.1.0.win3... ) and Python 2.5 ( http://www.stephanebrunet.net/gtkglext/pygtkglext-1.1.0.win3... ). Building it from source seems like a daunting task. Various build system utilities have to be installed first, such as MingW + MSYS, and pkg-config (which seems incredibly difficult to set up correctly on Windows). Also, it appears that the default maketools installed with MingW are too out-of-date to be used to build it. So far, upgrading MingW's autoconf / m4 / other tools is proving to be difficult at best. So at this point I'm planning to downgrade from Python 2.6 to 2.5 to save time, since it's basically impossible for non-ninjas to install or build PyGtkGLExt on Windows. I guess sticking with older versions is sometimes beneficial...
Besides that, from a practical standpoint I'd guess the Unladen-Swallow team wants a codebase on more solid footing, like the mature 2.6.x series, as opposed to Python 3.x which is in a greater state of flux.