The Semantic Web has been the fad of the moment for some years now, and if I was a major platform vendor I would at least make sure I had my toes in the water to support development of applications that wanted to interoperate with it. However, Microsoft seems to be mostly ignoring it, in terms of providing tools and libraries.
If you want to use the latest and greatest semantic web standards for representing ontologies (OWL and relatives), you need a package that can deal with the file formats and more importantly implement the reasoning and inference capabilities implied by this kind of representation. There are a few serious open-source packages that can do this, but they are all in Java or some other non-.NET language. If you want to work on Microsoft's platform, there is a nice package called SemWeb, but it operates only at the RDF-triple level, and is pretty much a one-man hack rather than a major research effort.
When I queried Microsoft about this, they basically said they don't care and I should try to use IKVM to run the Java packages on .NET. IKVM is a very cool hack but that doesn't sound very realistic given the bleeding-edginess of this technology and the need to debug across the API boundaries.
C'mon guys, aren't you about embrace-and-extend? Maybe the problem is they have their own splufty new language-integrated-data-access thingy (LINQ) and see the semantic web as something of a competitor/distraction. Maybe the whole flavor of Semantic Web is too cross-platform for Microsoft (but they took up the non-semantic Web, and XML, this is just the next phase).
So this entry isn't all kvetch, here's a useful compilation of semantic web toolkits for a variety of languages/platforms.