Sure, it would have been nice if Nokia was able to retain the advantage of a modern and proprietary OS. The thing is that their existing OS sucked, and Meego was turning out to be uncompetitive.
Of the choices Noika had in front of them, I think they took the best one available. Better than becoming "just another Android vendor".
What third-party developers are supposed to do is not clear. Will all today's applications break? Will there be a legacy runtime? What source-conversion tools will be available? Even key Nokia sources don't know the answer to these questions yet.
This is a silly statement. Windows Phone 7/7.5 are based on .NET, and Windows Phone 8 will be based on .NET. Sure, the underlying runtime may change, but it doesn't look like anything written for Windows Phone 7.5 will require anything more than a recompile (and I'm not certain that it would even require that). I'm sure it will still compile down to IL, and all of the APIs that were available in 7.5 will continue to be available in 8.
Sure, the OS kernel may change. But that has been abstracted away from the .NET developers.