Let me try to answer your questions.
We are not prioritizing upgrade the SDK to .NET 4, because there is a fully valid work around using the tools in .NET (Using VS directly or WSDL - there is a little trick with allowing cookies though).
Actually, I have talked with multiple customers who prefer to work that way. Thus, we would prefer to write better examples on how to build integration using the different tools on the .NET platform than maintaining the SDK.
Regarding replacing the SOAP API this will not happen anytime soon. It is true that we are looking at building a REST API, but this does not mean the SOAP API will disappear anytime soon, and the SDK will not work with the REST API.
Most likely when the REST API is exposed we will run with 2 API's which are equally valid from a business perspective, but the REST API will be easier to integrate against, especially for those where the SOAP is not that well supported in the language and/or framework they are using. When the REST API is ready and fully functional is another matter (I simple do not know).