• Backup everything on all computers you will be syncing. If you have a Time Machine backup do that. Also do individual backups of iCal and Address Book. In iCal use File > Backup iCal. In Address Book use File > Export > Address Book Archive. To backup Apple Mail navigate from your home folder (where you have Desktop Documents Downloads etc) to Library > Mail and then control-click (right-click) on the Mail folder and say "Compress Mail" from the pop-up menu. This will make a compressed copy of the Apple Mail folder that can sit there as a backup while the "in-use" Apple Mail folder is unaffected.
• Determine which of your computers has the most up-to-date information and plan to have this information overwrite the other computers. ie: we want to have the iCal calendar from one computer overwrite the iCal calendar on the other computers. We will do this will all sync info: Address Book Mail signatures etc. The goal here is to ensure that all computers are starting out with exactly the same data.
• Working from the computer with the most up-to-date information go to System Preferences > MobileMe • Go to the Sync tab in MobileMe preferences and then click the Advanced button
• Click the button to Reset Sync Data
• Leave the Replace menu at All Sync Info
• This part is very critical – we need to make sure the orange arrow is going from the computer to the cloud. Look for and verify the wording in this window says "On MobileMe with sync info from this computer."
• Click Replace
• Once this replacement sync finishes I like to have MobileMe do a couple of normal syncs just to make sure everything is running smooth. Remember that MobileMe sync is always between your computer and the cloud the computers never talk directly to one another.
• Now we can move to the other computers. The next time they do an automatic MobileMe sync it should present a dialogue notifying you the sync has been reset from another computer. I prefer to go straight to System Preferences > MobileMe > Sync > Advanced here and click the Reset Sync Data button.
• On the computer we intend to overwrite – make sure the orange arrow is pointing from the cloud to the computer and the wording says "On this computer with sync info from MobileMe"
• Click Replace. This will download the iCal etc data that you just "pushed up" from the first computer to this second computer you’re working on and replace the iCal data on it. Now we have two computers with exactly the same iCal data and MobileMe likes this.
• After having done the reset syncs the next syncs will be two-way automatically. This should get everything cleared up and syncing properly.