Anyone can play in Microsoft and Apple's OS garden. The major difference in Apple's mobile garden is that they strictly control who can enter AND what you can do once your in it.
>Update: Microsoft wanted us to clarify that enterprise customers will be able to deploy apps to employees outside the consumer-facing Marketplace -- details on that will be released in the future.
So you'll be able to do whatever you want if you're willing to pay, which is pretty much open in the traditional Microsoft sense.
This is exactly what Apple allows you to do, now, as well. If you want to go enterprise, you pay more (as you will for MS), and you can deploy to 100,000s of devices.
It looks so much like MS just took Apples model for Apps. MS have added nothing.
Replay the comparison, but versus Sony and Nintendo.