One of the issues we’ve come across when helping customers plan with the transition to Office 365 is the issue that once Active Directory Synchronization (DirSync) is turned on – it can’t be turned off.
As of October 17 (the same day Windows Intune 2.0 was released – what a day for cloud!) you can now disable DirSync using PowerShell.
You can read more here: http://support.microsoft.com/kb/2619062/en-us