Using Railroad and Graphviz
Blog update (10/22/2007): Thanks to the comment posted by Blog de Kilgore, I tried Railroad (instead of using the below method with starUML) to visualize my RoR app. Very easy to setup and use (see examples on web page).
Here’s a quick command to create a .png graphic file of your models:
railroad -a -i -M | dot -Tpng > models.png