Back in 1999, before I switched entirely over to a laptop, I had wanted to try multi-direction syncing of my Maildirs for use with mutt. I built a program that mostly did it, but in the end decided to just switch entirely to a laptop and rely on having my primary data with me all the time. rsync is a good one-way synchronization, but not 2 or more ways. By that, I mean reading mail on both systems, and having them both end up in the same, sane, state after the sync.
More recently, I've come across some other solutions that might have allowed for this. I haven't used them, but here's a set of links to some of them.
Coda is a file-system which can be run across multiple machines, with multiple writers and disconnected use. When a disconnected system gets connected again, synchronization and conflict resolution will bring all connected copies back into sync.
* tsync is a program from the Google Summer of Code project, and is designed to be a multi-way rsync. Multiple machines making changes, and resolving them amongst themselves. It looks to be pretty early in the development, but may be something to watch.
* Unison is another program but is limited to just two-way synchronization. For a laptop and a desktop, this may be ideal.