Here's an attempt at answering both of your questions.
What does it take for an existing .NET Developer to start developing for the Windows Phone? You can start developing for Windows Phone by simply downloading the free tools from http://developer.windowsphone.com
When can a developer start developing for the Windows Phone? Anytime... the tools are already available and you don't actually need hardware because of the built in emulator.
When will a developer be able to get their hands on a phone to do testing? Devices are just now starting to roll off the assembly lines. If you have a complelling application, simply contact your local Developer Evangelist and we'll be able to at least "test" your application of one of our phones. Some developers will be receiving phones very soon, however, with general availability within the next few months.
Can you share any reasons as to why a developer should focus on the Windows Phone instead of the iPhone/iPad? The Windows Phone 7 operating system is a completely new concept in the way a phone should be. The ease of development, the potential profits of applications and the large distribution model all enable to make a developer quickly make money. In addition, Microsoft doesn't have as "restrictive" a model as other devices, so you're free to build any kind of application (within reason) and sell it, give it away, or even use one of the other business models, like the "freemium" model for generating revenue. As a side note, we're actively soliciting cool apps that we can use as showcase apps for launch. If you think your app fits that kind of role, please feel free to contact your local Developer Evangelist to get your app in for consideration.
For someone new to phone development, how different is developing for the Windows Phone 7 Series from developing for Windows Mobile 6.5? Actually, there are some major differences and some major similularities. First off, it's still C#, so your business logic doesn't change. However, since it is an entirely new platform with a new UI paradigm, you will find that it is completly different. However, with that said, it's Silverlight, so if you know WPF or Silverlight already, you already know everything you need to build applications for Window Phone 7. In fact, you don't even have to be a programmer to write cool apps, because Expression Blend for Windows Phone will allow you to do very nice "drag and drop" development of apps for it. If you're already familuar with writting Windows or Web apps with Visual Studio, you'll also feel right at home writing apps for Windows Phone 7. All of the core concepts apply. If you want, you can even get your hands a little dirtier by digging into XNA, our game creation tool set, that will allow you to build very high performance 2D and 3D games and applications. In fact, the same games you write for the XBox 360 and Windows and easily be modified to fit the screen for Windows Phone 7.
As for your original pricing question, I'm not sure there is much we can do to help that out because that is local market based, not something that Microsoft does. Our OS licenising to hardware vendors is pretty consistent across the world, what they decide to sell their hardware at is based on their local conditions. I'm curious as to where you're seeing pricing already for the Windows Phone 7, as I'm unaware of any company who has it listed yet, since we haven't released anything to manufacture yet.
Lastly, what to expect from Windows Phone 7. Well, it's a completely new paradigm in the way a phone works. One of the biggest changes you'll see is how "connected" it is. Every app has the opportunity to participate in services from the cloud. In fact, most of our apps that are built in are built around that aspect. We rely heavily on services to provide a unique integrated experience to the user which no other device comes close to doing yet. Another aspect that is unique is the aspect of Live Tiles on the Start screen. These Live Tiles actually show you relavant "glancable" information as to what is going on. For example, if you look at a standard (US Based) Apple iPhone's weather app, the icon shows you 73 and Sunny when in fact, that is not true most of the time. On Windows Phone 7, the weather application can show you exactly what the conditions are based on the location of the device, without the user needing to launch an application to get it.
Same as cell phones, I'd guess. I used to love wifi for phone calls, before unlimited rates became realistic. If mfg's allow calls and data, they'd have a winner, as they wouldn't charge as much. (WIFI was FREE. The plan was a few dollars a month)
The only drawback was the phone. It roamed for a better source every 5 minutes, and you couldn't turn it off. Real bad when using WIFI with a strong cell tower nearby.
Used to drop calls every time there was electrical interference in the office (2400mhz Cordless Phone. MFG REMOVED)