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.