[CentOS] Nautilus won't start

I've been running CentOS 5 since release on a couple of servers with few problems. Last night I tried it as a workstation for the first time. It booted up fine and I could log into X. I then ran pup to install all available updates and shut down afterwards to load the new kernel. (I use yum on the servers.) This morning I start up on the new kernel and after logging in at the Gnome prompt, I get a message that Nautilus can't start.

I'm not an X guru, being mostly a server driver over ssh. What's the best way to debug this?

(I have lots of experience debugging, just not with X.)