Inside Mac OS X Snow Leopard: Exchange Support. Here’s a key statement:
Apple did not license Microsoft’s Windows-only “Exchange Active Sync” software; it merely licensed the rights to implement a compatible EAS conduit with Exchange. Apple owns the Snow Leopard software that talks to Exchange.
My experience has been that the Exchange support built-in to Snow Leopard is an excellent, first-rate alternative to using Outlook on Windows. All the essential features are there, and they work very well.