I have been racking my brains and getting frustrated with Mobile Me for a couple of weeks now, not able to find anything on the internet to aid in my resolution of iDisk not being able to sync, and certainly not syncing my offline files and folders, on my two computers.
I now have a solution. Albeit a rather odd one.

With iDisk sync switched on, and you are constantly receiving the message "idisk not in sync", follow the following steps:

1. Go to MobileMe under System Preferences.
2. Under the iDisk Tab click “Stop” to disable iDisk syncing. This will create a local copy of your files on your desktop called “previous local idisk”.
3. Next, go to the Account tab and click Sign Out.
4. Next, sign back into your Mobile Account with your username/password.
5. Go back to the iDisk tab and Turn On iDisk Syncing.
6. Now the iDisk icon should be back on your desktop. If you open it, it should start showing that it’s syncing. It may take a while depending upon how many files you have on your iDisk.

You may have to manually put any recently updated or created files from your “previous local idisk” into your iDisk.

Hope it works for you too.