You can very easily plug a mac in to an exchange based organisation. I know I have plugged one in to what was the largest exchange install outside of Microsoft's own campus. You can very easily make it map on to Active Directory, mount your Active Directory home folder and even use all of the exchange email and calendar functionality using office for mac. The reason organisations aren't switching from xp is because they trust it, its stable and it can do everything they need their users to be able to do.