Neal:
Comment added.

regrtest -R and -u all report no problems specific to this patch.  A clean build of trunk does show problems without it, with test_structmembers, test_tcl, test_telnetlib, test_unicode_file, and test_uuid.  At this point I've seen so many weird failures that I think I've started repressing them. ;)

When no stack frame is available warn() falls back to sys as the module.  Since I'm trying to avoid claiming a warning came from a different file, I made it appear to come from <no context> instead.  Looking at it now though I wonder if I should go a step further, not having a __warningregistry__ or module at all for such warnings.

Guido:
I see what you mean about stale co_filename attributes.  I'm not sure what the best fix is now.  I'm not exactly enthused at having to learn how the import mechanisms work. ;)
File Added: python2.6-warningfilename5.diff