OK, this hinges around the idea of post-modern languages, an idea from Martin Fowler. Basically a language like Smalltalk tries to control everything, whereas a post-modern language operates with the assumption that other languages will also exist. Watch Fowler's RailsConf keynote to really understand.
Anyway, I was reading the new TextMate book while stuck in an airport, and it turns out that TextMate has a footnotes plugin (by the extremely smart Canada Duane) which gives you functionality very similar to the functionality which renders a Web app written in Seaside into an IDE for itself. You get links in your Web app which you can click to automagically open the relevant code in TextMate.
What Seaside has is definitely more sophisticated than opening a file in a text editor, but TextMate is an absolutely beautiful text editor, extraordinarily powerful, redefining the category, and footnotes is certainly an excellent plugin. In fact, it's damn near the coolest thing ever. Smalltalk's self-contained nature is usually seen as a plus, but the diverse Unix environment makes similar features possible in a very different way.
Update: another tool for setting up a seamless Rails/TextMate "IDE".