I recently installed Centos 5.3 on my Lenovo T400, and everything pretty much worked except for the wireless connection. I had a hard time finding instructions on how to get it working but this blog entry was most helpful.
First make sure the RPMForge repo is installed
rpm -Uhv http://apt.sw.be/redhat/el5/en/x86_64/rpmforge/RPMS/rpmforge-release-0.3.6-1.el5.rf.x86_64.rpm
yum clean all
Next install the driver for the PRO/Wireless 5100 AGN[Shiloh] card from Intel
yum install iwl5000-firmware
The wireless device should now be working, and you can enable the NetworkManager to start using it
chkconfig NetworkManager on
service NetworkManager start
and you may want to save yourself some wait time at boottime by disabling the network since you don't need that anymore
chkconfig network off
Next you need to configure your wireless connection settings by going to System > Preferences > More Preferences > Network Connections
Select the Wireless tab and click on Add, and fill out your connection settings.