If Microsoft really were to "love Linux," this would happen. Feel free to gauge their marketing/PR sincerity in light of this. This and Office are really the only 2 things Linux really needs from Microsoft. Everything else is taken care of. I mean, SQL Server? That's fine, but, if you really needed a terribly-expensive database server running on Linux, we've had Oracle for, like at least a dozen years now.
I wouldn't hold my breath. I'm guessing this is more tied to the .Net moves to Linux and getting their full web stack on Linux. Though if their long term strategy is to give up Windows Server, then everything may come to Linux.
IIRC Exchange depends on Active Directory. I would've thought porting AD to Linux would be a huge undertaking, but then again I would've thought the same about MSSQL, so like you say maybe it's not so insane.
I don't think Exchange uses any interface other than LDAP to hit Active Directory, so I'd think there's at least a fighting chance that the AD requirement could be satisfied by other LDAP servers.
I can't speak to the IIS integration anymore-- I quit following that at a low level back in the Exchange '03 timeframe. I would suspect there's a clear line between IIS and Exchange, however, that could be used to facilitate an adapter layer for a flexible HTTP(S) server like nginx. The SMTP, POP, and IMAP services, though, would probably involve a lot of re-implementation.
