Dear All,

Recalling a previous problem where yum was too broken to update something on 
an RH box, but a colleague found that up2date worked, the same trick was 
tried here, & seems to have fixed yum.

root@bf41> yum check-update
'import site' failed; use -v for traceback
There was a problem importing one of the Python modules
required to run yum. The error leading to this problem was:

    No module named yum

[ snip - as before ]

root@bf41> which python
/usr/bin/python
root@bf41> ll /usr/bin/python
-rwxr-xr-x  2 root root 5396 Mar 14 17:30 /usr/bin/python*
root@bf41> file /usr/bin/python
/usr/bin/python: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV), for GNU/Linux 2.2.5, dynamically linked (uses shared libs), stripped
root@bf41> up2date python
0.  debug              No 
1.  useRhn             No 
2.  rhnuuid            5552bee2-d6e0-11da-8c41-0002b3e739c7 
3.  isatty             Yes 
4.  showAvailablePacka No

[ snip - long list ]

Enter number of item to edit <return to exit, q to quit without saving>: 
Your GPG keyring does not contain the Red Hat, Inc. public key.
Without it, you will be unable to verify that packages Update Agent downloads
are securely signed by Red Hat.
Your Update Agent options specify that you want to use GPG.
To install the key, run the following as root:
      rpm --import /usr/share/rhn/RPM-GPG-KEY

root@bf41>      rpm --import /usr/share/rhn/RPM-GPG-KEY
root@bf41> up2date python

Fetching Obsoletes list for channel: scientific-base...
Fetching obsoletes list for ftp://ftp.scientificlinux.org/linux/scientific/4x/i386/SL/RPMS/...
#######################################
Fetching Obsoletes list for channel: scientific-errata...
Fetching obsoletes list for ftp://ftp.scientificlinux.org/linux/scientific/4x/i386/errata/SL/RPMS/...
#######################################
Fetching rpm headers...
########################################
Name                                    Version        Rel 
----------------------------------------------------------
The following packages you requested are already updated:
python

root@bf41> yum check-update
Loading "kernel-module" plugin
Setting up repositories
sl-base                   100% |=========================| 1.1 kB    00:00 
sl-errata                 100% |=========================|  951 B    00:00 
Reading repository metadata in from local files
primary.xml.gz            100% |=========================| 493 kB    00:01 
sl-base   : ################################################## 1502/1502
Added 1502 new packages, deleted 0 old in 11.96 seconds
primary.xml.gz            100% |=========================| 179 kB    00:00 
sl-errata : ################################################## 415/415
Added 415 new packages, deleted 0 old in 4.36 seconds

So yum works again.

It is not clear what up2date python did - looking for things with that
timestamp in /usr, /var, /etc shows some in /var/lib (/var/spool/up2date 
of course); /var/log/up2date just says "up2date availablePackageList from
network2".

-rw-r--r--   1 rpm      rpm        331776 May  3 08:24 ./lib/rpm/Providename
-rw-r--r--   1 rpm      rpm         16384 May  3 08:24 ./lib/rpm/Installtid
-rw-r--r--   1 rpm      rpm         98304 May  3 08:24 ./lib/rpm/Provideversion
-rw-r--r--   1 rpm      rpm         45056 May  3 08:24 ./lib/rpm/Name
-rw-r--r--   1 rpm      rpm         12288 May  3 08:24 ./lib/rpm/Group
-rw-r--r--   1 rpm      rpm      34594816 May  3 08:24 ./lib/rpm/Packages
-rw-r--r--   1 rpm      rpm         12288 May  3 08:24 ./lib/rpm/Pubkeys

These are all Berkeley DB, so one/some of them might've been corrupt.
Thanks to all for support & help. 
It would be useful to know how to reproduce the original problem & investigate
exactly what fixed what... someone apply for a research grant for that!