As some of you may or may not know Office 365 DirSync or Windows Azure Active Directory Sync tool is the proffered method for syncing your on premise Active Directory on premise to Windows Azure Active Directory, which is a required step for single sign on to Office 365.
One of the limits of the existing DirSync tool is that you can only sync from one forest. So customers that may have multiple forests could not use DirSync to synchronise more than one Active Directory forest into a single Office 365 tenant. Lifting the hood on DirSync shows you that it is using FIM as the engine to sync to the cloud.
If you are a customer that wants to sync more than one Active Directory forest to your tenant, you can now use Federation Identity Manager 2010 R2 using the Windows Azure AD connector for FIM to overcome this deployment blocker.
More details can be found at the following URL: