If yum (CentOS) gives you the below error (or similar), you probably have a i386 or i686 python (or other) package installed on a x86_64 machine.

[root@earthgecko ~]# yum update
There was a problem importing one of the Python modules
required to run yum. The error leading to this problem was:
   /usr/lib64/libnssutil3.so: undefined symbol: PL_ClearArenaPool
Please install a package which provides this module, or
verify that the module is installed correctly.
It's possible that the above module doesn't match the
current version of Python, which is:
2.4.3 (#1, Nov 11 2010, 13:30:19)
[GCC 4.1.2 20080704 (Red Hat 4.1.2-48)]
If you cannot solve this problem yourself, please go to
the yum faq at:
  http://wiki.linux.duke.edu/YumFaq
[root@earthgecko ~]#

Try looking for any related i386 or i686 packages and remove them, SIMILAR to this, this is an EXAMPLE only! You may have different packages, etc.

rpm -qa | grep python
rpm -qa | grep python-devel
yum list | grep python
yum remove python-devel.i386
yum list | grep dbus-libs
yum remove dbus-libs.i386
yum update
yum list | grep glibc
yum remove glibc.i686
yum update

initial.error

Would probably be along the lines of:

--> Finished Dependency Resolution
glibc-2.5-49.el5_5.7.i686 from installed has depsolving problems
  --> Missing Dependency: glibc-common = 2.5-49.el5_5.7 is needed by package glibc-2.5-49.el5_5.7.i686 (installed)
dbus-libs-1.1.2-14.el5.i386 from installed has depsolving problems
  --> Missing Dependency: dbus = 1.1.2-14.el5 is needed by package dbus-libs-1.1.2-14.el5.i386 (installed)
python-devel-2.4.3-27.el5_5.3.i386 from installed has depsolving problems
  --> Missing Dependency: python = 2.4.3-27.el5_5.3 is needed by package python-devel-2.4.3-27.el5_5.3.i386 (installed)
--> Running transaction check
--> Processing Dependency: dbus = 1.1.2-14.el5 for package: dbus-libs
--> Processing Dependency: glibc-common = 2.5-49.el5_5.7 for package: glibc
---> Package kernel.x86_64 0:2.6.18-194.11.3.el5 set to be erased
---> Package kernel-devel.x86_64 0:2.6.18-194.11.3.el5 set to be erased
--> Processing Dependency: python = 2.4.3-27.el5_5.3 for package: python-devel
--> Finished Dependency Resolution
glibc-2.5-49.el5_5.7.i686 from installed has depsolving problems
  --> Missing Dependency: glibc-common = 2.5-49.el5_5.7 is needed by package glibc-2.5-49.el5_5.7.i686 (installed)
dbus-libs-1.1.2-14.el5.i386 from installed has depsolving problems
  --> Missing Dependency: dbus = 1.1.2-14.el5 is needed by package dbus-libs-1.1.2-14.el5.i386 (installed)
python-devel-2.4.3-27.el5_5.3.i386 from installed has depsolving problems
  --> Missing Dependency: python = 2.4.3-27.el5_5.3 is needed by package python-devel-2.4.3-27.el5_5.3.i386 (installed)
 You could try using --skip-broken to work around the problem
 You could try running: package-cleanup --problems
                        package-cleanup --dupes
                        rpm -Va --nofiles --nodigest