I'll pass on being an expert but thanks anyway!
I set out to achieve exactly what Ausatco wanted some time ago but ran into all the difficulties that have been aired here. That's why I settled for the low-level agricultural method I now use.
Then I gave up trying to improve it. Isn't it always the case that we stop refining a system as soon as it becomes acceptable?!
The more sophisticated and automated a system, the more fragile, inflexible, and error prone it becomes. If you knew how, you could write a script to do all this, but there has to be a good measure of operator involvement to avoid the risk of over-writing new address books and folders with old ones.
So, it's easy to synchronise messages on the fly as we have agreed. It's actually not difficult to synchronise folders too using File Management techniques with Windows Explorer.
When both (or All) PCs are home on the Network do the following.
In Win 2000 and XP on the Master Machine go to:
C: \Documents and Settings\Administrator\Local Settings\Application Data\Identities\{.........}\Microsoft\Outlook Express; where the 32-bit string in the curly brackets is your Main Identity. The file "Folders.dbx" contains the info you need.
Copy this to the equivalent location on your "Slave" or Client (after backing up the existing one!). You'll over-write the existing Folders.dbx.
The next curious step is mandatory. Open OE; you'll see any new folders added to the list but the the corresponing files won't exist until you open them quickly and close them again.
So now you can do a bulk copy of all the files in your Master folder above to over-write any new empty files in the Slave(s).
Rules
If you make extensive use of Rules you can synchronise where all incoming messages go in normal use. But typing more than a few Rules into a succession of machines is very boring indeed.
Rules are held in two places in the Registry (Current User and Users). But you can't simply export the Rules keys from the Master Registry and import them directly into the Slave's Registry because the 32-bit identity strings will be different.
What you can do - and I think you only need to do this once to the "Current User" key is export the Master's Mail Rules keys and use Notepad (Wordwrap must be OFF) to substitute the foreign (Master Machine) identity string with the slave identity string. That only takes a moment. Then import the amended Registry key to the slave machine and the Rules are now identical to the Master's.
I know this is a bit geekish but it does get you what you wanted. Oh, and do back up all Registry Stuff before altering it!
After this, synchronising Address Books will be a doddle; just use Windows Explorer to over-write the Slaves' with the Master's. Its in "C: \Documents and Settings\Administrator\Application Data\Microsoft\Address Book".