See the thread starting at http://mail.python.org/pipermail/pythonmac-
sig/2009-December/021907.html for full details.

It appears two vestigial gif files included in the 2.6.4 OS installer 
are being installed under corrupted file names by the OS X Installer.app 
under 10.3 and 10.4.  While the error is silently ignored on 10.4, it 
apparently causes the whole install to fail on 10.3.  The problem does 
not seem to occur when installing on 10.5 or 10.6.  It is also not 
limited to that installer image: I was able to reproduce the problem 
with an installer I built on 10.5.  However, an installer built on 10.4 
from the same source snapshot seems to have the correct file names.

Without knowing exactly why those two file names, and only those two, 
are corrupted, it seems risky to let this seemingly minor problem go 
unresolved.  And it apparently prevents 2.6.4 from being installed on 
10.3, a more serious problem.