We keep hearing that this is impossible due to restrictions from Microsoft and ActiveSync implementation. BUT, on a Windows Mobile device users CAN access multiple Exchange accounts. All they need to do is a registry edit, and voila it works. see this url
BSKB-000003: Synchronize a Windows Mobile 6 device with multiple Exchange mailboxes - Bitshift Knowledge Base
Can someone figure this out for the iPhone? I don't have a MAC, so I can't even look at the iPhone Developers stuff, but I DO have multiple MS Exchange accounts because I'm a consultant with multiple companies.