There was a post to the RubyCocoa mailing list about two weeks ago, talking about how to compile RubyCocoa for Universal usage. I noticed that one of the committers on the project added a configuration switch to do this about two days ago, so here is a combined installer for the latest versions of both Ruby and RubyCocoa (1.8.4 and 0.4.2 (+ extras)) for running on both PowerPC and Intel based Macs.
Please let me know if you run into any trouble using this package.