It took me a little to figure this out (including a visit to the #centos IRC channel on freenode.net), but I finally got the upgrade from CentOS 4.3 to 4.4 working.
My problem was that when I stuck the DVD in and booted, it got to a point where it went searching for a previous CentOS installation -- but could never find it! Grr. I didn't want to reformat my drive and start from scratch. But, I found this command that you enter before getting into the installation:
linux upgradeany(There is no space between "upgrade" and "any") That did it. When it went looking for a CentOS installation and it found my CentOS 4.3 installation. I'm upgrading my notebook as I post this. Peruse this info if you are also running into this problem. Update: After figuring out that little "upgradeany" thing, the upgrade was rather fast and painless. It took only 20 minutes for the installer to update the system. I had to plug my notebook into the wired network so that I could get the updated kernel (which I already had, but got downgraded by the installer). Once I had the updated kernel, my madwifi was working again. So, all is well. Kudos to the CentOS team for putting out a great release. Thanks also to Red Hat for putting out easy to rebuild SRPMs for the CentOS team. The graphics have been tweaked in this release and the grub bootscreen has a really cool looking graphic now. I think I'll even keep the CentOS as the Main Menu button.