We have a variety of folks using the TIBCO Ajax libraies and visual development tools with Ruby. If you're not familar wiht TIBCO GI, it just won "best open source ajax toolkit" from InfoWorld. See http://developer.tibco.com/gi.
Anyways GI integrates with anything that can service up structured data over HTTP -- usually XML or SOAP data. You can use Ruby to expose information as XML or SOAP, then build a GI Ajax client to that. GI's model is more a "ajax client/services" architecture rather than a "ajax-enriched html page" conceptually. So it's not for everyone, but its visual tools and more than 100 ajax components out-of-the-box make it a top pick for anyone building apps from resulable components and data services. Ext.js would be more like GI, though GI has much more available in it. GI is also BSD open source licensed whereas backbase is commerical and Ext.js is now requiring a GPL-based license I think. I hope that helps.