I'm a bit late with this, having been scooped by Ruby Inside even though I knew about it, but the founders of Bitscribe have a new company, Heroku, which looks to be quite nifty. Essentially it's a Web-based IDE and deployment system for Rails apps which incorporates an editor, log files, and the console, and could very possibly at some point support a debugger as well (since ubercoder Adam Wiggins, who wrote Gyre, is a key member of the startup).
Obviously this picks up a thread from Seaside's thinking and pursues it; I think both Seaside and Heroku are operating from the perspective that deployment is a logical thing to include in an IDE, and if the Web is truly the new development platform, then an IDE should run on the Web. The app's still in its early phases, and private beta is still on its way, according to Ruby Inside. However, I got to play with it because I know these guys, and it's pretty cool.
I don't think I can say too much about this next bit, but there's another company out there pursuing a similar line of thinking from a different angle. Heroku is really onto something. There was a popular idea in the late 90s that the Web should replace the desktop; destroying this movement was Internet Explorer's primary goal and raison d'etre. The fact that some people on the Web today might not even remember or recognize that idea shows that IE made some considerable headway there, but projects like Seaside and Heroku show you how it can be true. Heroku's cool now, but fast-forward a couple years and assume Rubinius performance catches up to Ruby performance; a Web-based Rails IDE starts to look very promising.