The DST patch for Exchange breaks the ability for a user with Full Permissions from doing a SendAs for the mailbox. You have to manually assign the privilege.
Fix: http://msexchangeteam.com/archive/2006/01/13/417440.aspx. This goes for Blackberry and Goodlink, but setting the SendAs permission for a user is the same, regardless. Simply apply it to the user in question rather than a group.