I have just installed Exchange 2010 and moved some mailboxes. If you go to the the https:\\exchange2010\owa
for those mailboxes it works. If someone goes there and their mailbox is on an exchange 2007 they don't get redirected to that server, and it says they are supposed to. They get the message " Your request couldn't be completed because no server with the correct security settings was found to handle the request. If the problem continues, contact your helpdesk.".