After logging out of your MobileMe account, you may find that your iOS device no longer automatically syncs with your Mac, even after you log back in to MobileMe. This very thing happened to me this weekend after I logged out of my MobileMe account to troubleshoot an issue with iWeb.
When you sign out of your MobileMe account on your Mac (System Preferences > MobileMe > Account) your iPhone, iPad, or iPod touch will no longer be syncing automatically. For example, adding an iCal event on your iPhone will not add it to your Mac, even after you've signed back in to your MobileMe account.
Obviously, this can cause problems if you are not aware this occurs. Users can manually sync with their MobileMe account (after they have signed back in) by clicking on the sync icon in their menu bar and choosing "Sync Now." This should get your contacts and calendars up-to-date, but it will not resume automatic syncing.
Resume automatic syncing with your iOS device.
To resume automatic syncing with your iOS device, open System Preferences > MobileMe. Be sure you are signed in (Account). Click on the Sync tab.
Check the box for Synchronize with MobileMe and choose Automatically from the drop-down menu. Click the Sync Now button.
If you get the following message, choose Merge Information:
Once you've synced MobileMe with your Mac, automatic syncing should again be functional for your iOS devices. Do you sync your iOS device over the air with MobileMe? Why or why not? Let me know in the comments!