I think that my requirement is fairly common but I'm struggling to find an elegant solution whereby I can keep my mail systems up to date across two different Apple machines; an iMac and a MacBook Pro (strictly speaking it's three machines, but as the Hackintosh is still running under Leopard and the others under Snow Leopard, I'll not include that in this problem discussion). Currently my iMac acts as the primary Mail machine, and all mail is delivered there from my ISP and immediately and automatically deleted from the server. If I'm on the road for more than a day, I'll close Mail down and copy the entire "username"/Library/Mail folder onto the MBP. The laptop then becomes the primary Mail machine until I return home. I then reverse the procedure to revert back to normal. The key to this approach is to make sure that only one machine is the active Mail computer at any one time, and on occasion I forget that I'm not supposed to start Mail up on the "wrong" machine until all necessary transfers have been completed. There are mechanisms to avoid this, like keeping the server copies in place during the trip, but even this requires changes to preferences which are easy to forget.
I recently started using SugarSync to keep my work folders in sync across desktop and mobile devices. I've configured it to work with Things and SketchBox most recently I've solved the problem of managing local drafts in MarsEdit across a network. SugarSync is a relatively inexpensive way to keep files and folders synchronised. I have subscribed to the cheapest plan which gives me 30Gb of storage and allows me to sync across any number of machines. This basic plan costs $49.95 per year. A free plan also exists for users with more modest requirements and gives you 2Gb of storage and sync capability for two computers. A SugarSync app is also available on the iPhone for increased flexibility.
Given the success of using SugarSync with other systems, I decided to see if anyone had set it up to work with Apple Mail. My searches proved fruitless, and I summoned up the courage to act as a pioneer. I backed up my primary Mail system on a USB stick, and did an extra Time Machine backup on both desktop and laptop computers. I then configured SugarSync to manage the "username"/Library/Mail folders on both machines. This folder weighed in at about 600Mb, including all subfolders, and contained about 10,000 items.
For my first attempt I worked on the iMac first and SugarSync duly went off and started uploading all the data from my mail system. This took several hours at an average upload speed of about 0.35Mb/s. I then attempted to sync the laptop and was surprised to see that almost everything from the laptop mail system was also being uploaded and marked as such in the filename. I figured that this must be because the desktop and laptop were completely out of sync and was concerned that the duplication of critical files would cause Mail to have a nervous breakdown. I halted the process, reset both machines, performed a manual copy of the iMac mail folder to the laptop and started again. I left things running over night and in the morning was amazed to find that the same thing had happened. I attempted to start Mail on the iMac but it just spluttered and died requiring a Force Quit, and another reset of both Mail systems, after removing SugarSync from the equation.
I've not got enough technical knowledge about the internals of the Apple Mail system to understand what's going on, and why SugarSync was unable to create a single set of files which Mail could then read. And I'm not sure that I really want to find out. I'm definitely of the opinion that, in certain subject areas, a little knowledge can be a dangerous thing. This is certainly one of those subject areas!
So, SugarSync is not the silver bullet to solve this particular problem and I'm still on the lookout for that elegant solution to synchronise Mail across two Macs. If anyone knows how to do this (without setting up Mail servers) please let me know. I guess the easiest way is to keep all my messages on the ISP servers and once a week ensure that both machines are fully aligned before deleting everything off the server.
But that still requires manual intervention and may still not be foolproof. Or Ally-proof. And it certainly isn't elegant!