Thanks for the tip, but this does not work for me.
My impression is that Matt Parkinsons blog is (unfortunatly) wrong.
I am in front of my office iMac (10.8.2). I did change a rule, waited a while, and quit mail on the iMac.
Booted my MBP (10.8.2), opened Mail.app, and checked the rules list. The modification in the rule changed on the iMac was NOT there.
I created a new Smart mailbox, waited a while, and quit Mail on the MPB.
Checked on the iMac after opening Mail. The new Smart mailbox does not show up.
Since I am doing the "propagation of my rules" since quite some time via dropbox, I am also sure that the change dates of the three files MessageRules.plist, RulesActiveState.plist and UnsyncedRules.plist stays as is on one Mac even if rules are edited on another. The same conclusion results from searching (a copy of) the file MessageRules.plist using TextEdit to compensate for the lack of a find/search/locate function in rules with Mail.app.