I really like the work that's done with processing (http://processing.org) but I'm more exposed to the .Net world and would like to stay there. Is there a library or sub-language within .Net that is very focused on being easy to use for artists?
Processing and OpenFrameworks excel because it's so easy to create animations and other visual art from code without having to worry about the other bits.
I'm not sure if this is exactly what you're looking for, but you could check out Expression, which is a designer-focused tool from Microsoft. It is not free, though, and really more of an IDE than a language/framework. It is supposed to allow developers (coding the behavior/logic) and interface designers (coding the user experience/interface) to work together effectively.
there is XNA of course - a games library for C#. Although not quite as easy to use as processing it's not far off once you've overcome the initial learning curve. XNA has has much more powerful graphics than processing and uses a vastly superior IDE (visual studio) and gives you all the advantages of c# and .net.
Microsoft has come out with Vedea, which is they're actually billing as Processing for .Net It should be available in early 2010