Apple is really turning on the pressure by saying: Once you own a mac (which you need to in order to develop iOS apps) and you know Objective-C, and you are familliar with Xcode. Why not put in a little bit of effort and offer apps for OSX as well? You can make additional returns on your same basic investment.
Apple is now offering developers one development environment which allows you access to three of the five main screens in a person's life, computer, tablet, and mobile. The remaining two are TV and the windshield of your car. AppleTV doesn't support Apps yet, but you have to be a fool to think they aren't coming.
Google will most likely beat Apple to the windshield, but lets see if they can do anything compelling there.
The difference between Microsoft or Google offerings: Each of their solutions to the 3 screens in which Apple currently competes requires different development tools and know-how. XBOX apps are not Windows apps are not Windows Phone 7 apps. ChromeOS apps are not Android apps are not Google TV apps. [Although I'm expecting Google to pull an interesting move in this space and allow Chrome Apps to work on Android and GoogleTV, which would make Chrome Apps the most interesting for developers who want to maximize their screen reach.]
Apple is playing to developers in a completely different mode than their competitors are. Developer lock-in in iOS will continue at a nice clip, it's such a lucrative market when you already know that people who purchase Apple products have already proven their willingness to shell out cash for premium products.
This should at least compensate developers for the rigorous (to say the least) App store admission process. Which I think is a good thing. Apple is first educating developers to be responsible about the user experience for the software they release, and then is rewarding them for good behavior by allowing them access to a growing number of screens and customers who are ready to pay for responsible user experience.
It's a net win for the world in my opinion. Because there's plenty of room for Microsoft and Android to commercialize anyone who doesn't fit the Apple mold, but those competing products will still need to compete on usability and making it easy for developers to access their markets.