This has just appeared and is being spread across several peoples' blogs. I can't wait for this series to kick off (for the public) it looks like it is going to be good. At the moment they only have up an introduction which includes some good little pointers to why API design and forethought is good. From what I have read, it was a course for internal MS people but they are releasing some of the stuff (that they are allowed to) for us to listen to. Brad Abrams seems to be a good speaker...
It appears to be weekly and I am really looking forward to this.
MSDN .NET Framework Developer Center: Designing .NET Class Libraries