Fellow ZDNet Blogger Mary Jo Foley has it on good authority that iPhone will be compatible with Microsoft Exchange Server.
Word is that Apple make the announcement as soon as today that:
...it has licensed the Exchange ActiveSync licensing protocol. Via the licensing arrangement, Apple iPhone users will be able to connect to Exchange Server and make use of its wireless messaging and synchronization capabilities.
Mary Jo also points out that a number of phone vendors, including Nokia, Palm, Motorola and Sony Ericsson, already already sync with Exchange using ActiveSync.
Still no word on when it would happen or whether Apple is also speaking to Blackberry and Good, but the next few days and weeks will be interesting to say the least.
Update: I removed "Rumor:" from the headline after corresponding with Mary Jo and re-reading Walt Mossberg's iPhone review, which contains this line:
It can also handle corporate email using Microsoft's Exchange system, if your IT department cooperates by enabling a setting on the server.
Update 2: As fellow blogger George Ou points out Mossberg's passing remark isn't exactly confirmation that Apple will fully support Microsoft Exchange Server:
Walt's quote is WAY too vague to be used as proof. For all we know, that just means your IT department will have to turn on IMAP support and permit IMAP through the firewall. Even if Mary's source turns out to be reliable, you have no control over corporate data that way in case you need to do a remote wipe of data on a stolen or lost iPhone. Try explaining that one to your SOX or HIPAA auditors.