Microsoft to release Outlook for Mac, but why?

We read today that Microsoft (MSFT) is finally releasing an Outlook email client for Mac, according to TUAW and others. It's about time. Microsoft has specifically fenced out Apple (AAPL) users by making it just hard enough to run the Microsoft Entourage mail client on corporate IT systems. But why the change of heart? I can hazard a few guesses.

First and foremost, this is probably a precursor to releasing Office for iPhone. Microsoft announced a deal with Nokia (NOK) to put Office on the Finnish cell giants devices, indicating that Redmond is in the mood to port Office to mobile handsets running alternative operating systems (BREW in the case of Nokia). A jump onto the iPhone would be a logical next step. But it would be a pointless move if there was no viable Outlook client for Mac.