This is something I've been wondering since a) Apple included Exchange support in iPhone 2.0 and b) Apple included Exchange support in Snow Leopard.
It seems to me by doing this Microsoft are shooting themselves in the foot. I know a number of Windows Mobile users who have moved to the iPhone because of Exchange support. These users will likely never go back to Windows Mobile.
On the other hand I know a number of MDs who are now considering upgrading from Exchange 2003 to Exchange 2007 because Snow Leopard supports Exchange 2007 natively, and it means they can move away from Entourage.
Lock-in has always been a big deal for Microsoft, just look at how they pushed IE into the dominant position it is in today. By licensing ActiveSync they completely remove the requirement to use Windows Mobile, and ultimately will lose to Apple and Google in the mobile space.
I wonder whether it's just big company syndrome? The guys in Exchange are trying to increase sales and usage (maybe to curtail the openness that Google provide with Google Apps) but by licensing ActiveSync they're killing Windows Mobile.