Microsoft may (actually I think it will) in the future release the IE10 Chakra (JScript engine) as a stand alone module, like google V8 JavaScript Engine.
JavaScript (as JScript) is one of the original languages supported by .Net. Support was created before dynamics was available, so in that sence it is different from IronPython... If you need a JavaScript engine in .Net it may be enough for your needs - http://msdn.microsoft.com/en-us/library/72bd815a(v=VS.100).aspx.