[Yum] Yum broken after an update

[lists at natch.net (natch)]

I have a number of machines that I'm keeping updated with Yum.  One of 
them, a fresh install, ate itself after an upgrade.  When I execute YUM 
I get:

yum --version
Traceback (most recent call last):
   File "/usr/bin/yum", line 22, in ?
     import yummain
ImportError: No module named yummain

Any suggestions would be appreciated.

-
natch