I always see Ubuntu 8 vs Ubuntu 10 vs Ubuntu 11 and recently not any of the other distributions around the Linux kernel.
The problem for me is, that I am not really familiar with Debian based distributions and never used Ubuntu for anything

RedHat does a lot of custom kernel stuff as well, maybe the new CentOS 6 would be interesting to see?

How about a vps shootout between distributions on OpenVZ or something(there are even vps stages of Gentoo)?