matplotlib SVN trunk appears to be running backend_driver.py fine under Python-2.6 with Numpy SVN and TkAgg backend.
I had to make a handful of changes to remove deprecation warnings, none of which changes our "still supporting Python 2.4" policy. One change was required to pytz, which I've communicated upstream to its author.
In case anyone is wondering, there don't seem to be any measurable performance increases...
Science Software Branch
Operations and Engineering Division
Space Telescope Science Institute
Operated by AURA for NASA