E2K10 SP2: Thanks for the new custom attributes!
One of the nice new things that Microsoft introduced in Exchange 2010 Service Pack 2 did not make much news. There are 30 new extension attributes (aka custom attributes). These are msExchExtensionAttribute16 - msExchExtensionAttribute45.
Note, these are NOT editable from the Exchange Management Console nor the Set-Mailbox cmdlet, But, they are flagged for Global Catalog replication, they are assigned a mapiID and they can be added to the details templates!