Full Mailbox Permissions and Send As permissions are one of the essential things in daily operations, I believe and they are still there in Exchange 2010 too. Take a look at http://technet.microsoft.com/en-us/library/bb676551.aspx This article should help you understand how to do it.
Okay, dumb administrator question number 6,328.
In Exchange 2003 (yes, I’m dating myself, but my last employer used 2003), an administrator could assign a user as a ‘delegate’ for their mailbox, giving them a subset of permissions, including a modified “send as” permission that let the delegate send messages “on behalf of”.
Did this go away in 2010? Or did it get rolled into the “full access” and “send as” mailbox permissions?