My, what a coincidence, I've just made available my patched version of Modest that includes this functionality built-in :-)

Tinymail has in fact supported offline sync for IMAP folders since before it forked from Camel. But it's also been broken all that time, and there was no way to switch it on through Modest. So I added that. Not a lot of code really, although it was a pretty steep learning curve for me. Packages are available from here:

http://mat.exon.name/modest/

And the thread where I've been talking about it:

http://talk.maemo.org/showthread.php?t=55771

There are many limitations to this, not least that I have strong doubts this functionality will ever be accepted upstream and eventually make it's way into a new release. So it's definitely a good thing that the dovecot/offlineimap solution is also available.

In fact, the dovecot solution sounds spookily familiar to the solution I ended up with on my N810. But I hacked up my own perl script instead of using offlineimap. At any rate, sounds like you had an easier time building dovecot than I did under Diablo:

http://mat.exon.name/logs/maemo

This is progress!