I'm starting to dig Seaside - a Squeak based web app server - it uses continuations to basically stash a copy of the app in a given state mapped to various unique urls. Makes backtracking more straightforward to deal with (data persistence to a rdbms is another thing).
You might also be interested in looking at [link|http://www.plt-scheme.org|PLT Scheme], since IIRC they are the folks that Avi Bryant got the idea of continuation-based web apps from.