The problem is best described using example code (also see http://trac.saddi.com/flup/ticket/12#comment:3):

  * file foo.py: see attachment
  * file bar.py:
      import foo

Running foo.py directly produces the expected result (html output from cgitb), running bar.py outputs nothing as it seems to hang internally (only the thread which should print the html).
Moving all non-import into a function in foo.py and calling that function after the import in bar.py leads to the expected result, so it seems to be related to code which is executed while still in import and while using threads.
I don't think it is serious problem as it's only triggered in that unusual case.

I'm running python-2.4.4 on Gentoo Linux.
Linux tux 2.6.20-gentoo #2 PREEMPT Mon Feb 5 19:20:30 CET 2007 i686 AMD Athlon(tm) XP 2600+ AuthenticAMD GNU/Linux