OK I have been participating in Exchange 2010 beta testing and also Office 2010 beta testing, but I never heard that one of the most requested feature is actually available in Outlook 2010!
Below you see a screenshot of my accounts in my default Outlook 2010 profile.
I have 4 Exchange mailboxes from 3 organizations in my profile and 2 hotmail mailboxes.
Before Outlook 2010 you could add extra mailboxes, but only from the same organization and sent items were saved in sent items folder of the main mailbox.
Now in Outlook 2010 by default it will let you connect to 3 Exchange mailboxes maximum, in or outside default Exchange organization.
But Group Policy settings (GPO) for Outlook 2010 can change this maximum behavior from 1 to 15 (!).
This GPO set to maximum of 15 Exchange accounts, translates to this registry setting:
Windows Registry Editor Version 5.00
Other GPO settings are also available for forcing behavior.
Now I wouldn’t be happy with 15 mailboxes in my Outlook, but I can manage 3 or 4. I suggest using the favorites section for quick access.
When sending a new message you can choose which (Exchange) account to use.
Items you sent are saved in the sent items of your Exchange account
Thanks to Kees for pointing me to Jaap Wesselius article: http://ucug.nl/blogs/jaapwess/archive/2009/11/12/outlook-2010-en-meerdere-exchange-mailboxen.aspx