MobileMe is scheduled to be shut down in June 2012, and Apple has begun encouraging users to transition to iCloud. Email messages are going out this week that promote iCloud's services and provide a big 'ol blue button for customers to click and begin the migration process.
iCloud offers most of the services MobileMe customers are used to (keychain sync and iDisk are absent, for example) and, according to CEO Tim Cook, is the foundation for the company's future.
It's only February, so stragglers have some time, but not much. It's definitely time to start thinking about making the move.