Nope it does the 'on-behalf':
Definitely vote for:
Bug 22819 - reply to another user's message is always on-behalf-of Bug 19146 - send on behalf of for delegate access for ZWC Bug 27259 - Option for "On behalf of sent mail" to go to "on behalf of" user's sent items
This is the 2nd thread today where people are running into that on-behalf not doing what they want when...which may be fine if the accounts have a close relationship but there's definitely some 'gotchas' where personas still rule. (Though you don't want personas in your situation.)