From the Chandler email list, some references to the use of continuations in the Haystack system and the Python language...
In September, Hamish Harvey described some of the neat things about Haystack, including UI continuations. (Harvey's message was posted Fri, 26 Sep 2003; there's an article describing Haystack's continuations at http://haystack.lcs.mit.edu/papers/uist2003-uicont.pdf.)
I just wanted to add a reference to a tutorial article on continuations using Python: "Continuations Made Simple and Illustrated" by Denys Duchier at http://www.ps.uni-sb.de/~duchier/python/continuations.html.
I also want to second Harvey's enthusiasm for the idea. When the design work on CPIA gets into high gear, I strongly recommend reviewing this technique.