This has been discussed a fair amount, as well as replaying session transcripts, persisting key data structures easily, some other things i can't remember.

http://groups.google.com/group/clojure/browse_thread/thread/...

http://groups.google.com/group/clojure/browse_frm/thread/473...

http://stackoverflow.com/questions/3480377/clojure-namespace...

http://clojure-log.n01se.net/date/2010-01-16.html#i75