I maintain a URDF for the Create (which is functionally identical, unless you want to simulate the vacuum) here:
http://code.google.com/p/aptima-ros-pkg/
The package name is irobot_create_description and has launch files for gazebo in the 'launch' directory
There are still some TODO items like simulating the bumper and drop sensors, but if you just need odometry it should work fine.
UPDATE: Looking at the turtlebot_description package it looks like it was originally a fork of my URDF. I'll see if there's any additions they made that would be useful for backporting.