After the latest update to 11.28 stable, we began experiencing this issue on some CentOS 5.5 servers when trying to run the command: yum:

Code:
There was a problem importing one of the Python modules
required to run yum. The error leading to this problem was:

   /usr/lib64/python2.4/lib-dynload/datetime.so: failed to map segment from shared object: Cannot allocate memory

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, Jan 21 2009, 01:11:33) 
[GCC 4.1.2 20071124 (Red Hat 4.1.2-42)]

If you cannot solve this problem yourself, please go to 
the yum faq at:
  http://wiki.linux.duke.edu/YumFaq
it turns out the issue is related to resource limits set by cPanel under /etc/bashrc

To fix the problem, login to cPanel/WHM and search for the option "Fork bomb" , disable that option.

Good luck to all!