Back on topic: I think I won't switch to Celeste before the file format is not compatible with the standard unix stuff. Maybe there should be various backends? One for the monolithic message file, one for your "each message a separate file", one for "each message folder a file, nested folders are directories (my pref)" - and then, IMAP support would be cool, too :)
Directly accessing files that are in foreign formats is pretty awkward. It's a big design constraint, and it would put pressure on things like having a full index available for searching.
Directly accessing Unix mail files in particular is hard, because Squeak can't do Unix locking. This is a specific reason that POP is the main way Celeste gets email: POP works to and from any platform. Could you instead set up your Unix files to be accessible via POP (or later, IMAP)?
Anyway, I like the idea of having multiple *sources*. And IMAP should be one of them. Doing IMAP right would require an improved index file format, to avoid downloading the same message multiple times.
-Lex