GMail is not even close to a replacement for exchange in a large company. Small companies like mine can use it just fine but I can't see it working as well in a large company. In fact if I still had to write large quantities of mail (which I used to) I would still use outlook as a client as it has much more developed workflows and a richer set tools.
Outlook works just fine talking to Google Mail; our Windows users connect to Outlook. The rest of us Mac folks use Sparrow (which is not long for this world, unfortunately).
Everyone has iPhones, which work great with Google Mail and Google Calendar. I'm exceptionally happy that, for most small and medium sized businesses, there's no need for anything other than Google Mail.
I'm not entirely sure why "Enterprises" would need Outlook, but I'll cede that's a market I don't know very well, so there may be very good reasons for it.
