I have a user who is set up to send on behalf of several of other users. One works just fine, she can send on behalf, and everything is fine.
The second user, she can send, but it shows as sent by the user she is sending on behalf of (as if she has send as rights, which she does not), without any reference to her. I set up a test account, replicated the procedures to grant send on behalf settings and it works fine for the test user.
The third user, she has been made a delegate through Outlook 2007, and shows in Exchange 2010 as having the appropriate permission. When she sends on behalf of this user, it bounces back says she doesn't have permission.