Several customers have had some issues getting the atl1e driver to work correctly on Centos Linux 5.2 and 5.3. There is a kmod-atl1e package floating around some RPM repositories, but that doesn't always work right. After spending a fair amount of time helping customers work around problems caused by kmod (drivers are not in the normal place, updating a kernel breaks it and then it won't let you remove / reinstall, etc.) I found the easy way to get these drivers.
Start here. This gentleman was kind enough to rework the Makefile that comes with the atl1e driver source and post the new one with lots of fixes. If that ever goes down you can download it from us here.
Just untar it with 'tar xvf l1e.tar' and cd into the src/ subdirectory, and run 'make install'. The modules are now in the usual place, /lib/modules/<kernel-version>/kernel/drivers/net/atl1e.