A first hand look from the .NET engineering teams
Now that .Net 4 Beta1 is out, you'll see a number of posts on this blog covering new CLR features in CLR 4.
Grant Richins, a developer on our team, wrote a post on tail call improvements in CLR 4 at the CLR Code Generation blog. The post deals with JIT ETW Tracing in .Net 4 and specifically covers new ETW events exposed by the JIT. Prior to .Net 4, there was no easy way to determine why the JIT (or the runtime), did or did not inline or tail call a particular method. With these new events, this is now made very easy. You can find the post here.