I've been playing around in Ubuntu Server for the last couple of days, setting up a test sandbox of WordPress 3 to safely try upgrading UCalgaryBlogs. I've used Ubuntu before, but it's really feeling like a solid system, and the LAMP stack is fast. I'm only running it in a VM under VirtualBox on my desktop, but if I had to deploy a fresh server, I'd definitely be picking Ubuntu to run it. As a virtualized sandbox, it's a pretty great platform. And the ease of updating/configuring is pretty slick. Certainly more nimble than RHEL5...
Although, the native blog posting app is utter crap, compared to something like MarsEdit...