Xcode 3's language support for Ruby is somewhat broken. Fortunately, fixing it only requires a few small modifications.

One major problem, Xcode doesn't know what .erb and .rake files are.
There should be a file:
/Developer/Library/PrivateFrameworks/DevToolsCore.framework/Versions/Current/Resources/Standard file types.pbfilespec

Copy this to /Library/Application Support/Developer/Shared/Xcode/Specifications/, creating that directory if it doesn't exist. Editing the original works, but your changes will be overwritten if you update Xcode, better to just override the standard files.

Xcode 3 allows you to specify the syntax of a language with .xclangspec files, which contain information on the structure of the language that is used not only for syntax coloring, but also for populating the symbols menu, showing the currently selected block with the "Focus Follows Selection" feature, selecting the appropriate amount of text on a double-click, etc.

The POV-Ray syntax specification file is a little crude, work still to be done with it, but already a considerable improvement over the other options on the Mac for editing POV-Ray files.