It's now been more than 16 months since I promised to post about the improvements we made in the ThreadPool's concurrency control algorithm for CLR 4. Lesson: don't promise to write a series of articles unless you've got the time to do it!
Luckily, Erika Fuentes, who you may recognize from our Channel 9 video, has written an article on this topic for MSDN Magazine. Lesson: lazyness works!
If you'd like to understand more about how the ThreadPool works, please do check out Erika's excellent article, Throttling Concurrency in the CLR 4.0 ThreadPool. If you have any questions, feel free to post them as comments here, and I'll do my best to answer.