The newer, more full-featured kid on the embedding Maps-in-your-Rails-App-block has gotten an update. YM4R v0.5.1 was released a couple of days ago. It's now mostly a plugin, instead of a gem, since it includes a lot of Rails specific stuff. There are Ruby libraries for creating GoogleMaps tiles, and geocoding that are still in the gem, which is great too.
And I've even done my part to contribute by submitting my GeoRSS Simple Parser.