Today, Apple updated the MobileMe control panel for Windows. Update 1.2 "provides better support for syncing a large number of contacts (more than 2,000) and prevents syncing of data between Microsoft Exchange Server and MobileMe, a configuration that is not supported."
This control panel is required to use MobileMe syncing/iDisk on a Windows computer. This update requires Windows XP Home or Professional with Service Pack 2, or Windows Vista SP1 or later. You must have iTunes 8.0.1 or later installed.
You can download this update from the Apple Support website.