I wouldn't get to excited, I don't think its going anywhere.. theres just a lot of work that needs to go into a browser.. writing one in .net would be interesting from an extensibility point (perhaps, but still less so interesting than extensions in say firefox- which are much more approachable).
But I don't really see otherwise why..
The reason is simple. Anyone could use the same core to build browser ontop of .Net. Any machine supports .net platform would be able to run the program easily. For example, run you own version of browser on Win Mo, on Win CE based GPS system, on many other Win CE bases systems.