This post by Brad Abrams announces the availability of the first in a series of framework design presentations. Realize that these presentations were originally targeted at an internal Microsoft audience, so keep that perspective in mind as you view the material. Not only will you learn what it takes to design a consistent and usable framework, but you'll gain some insight into how we do things internally.
The first presentation is titled Setting the Stage and is described as: "Set up for the course, generally introducing the practice of API design. Topics covered include terminology, first principles, and why API design is a crucial thing to get right (the first time!)." It's available at 56k, 110k, and 300k. If you'd like to discuss the material of ask questions of Brad, he'll be available for a chat on 1/19 at 3:00pm PST.
Although I haven't taken the training myself (yet!), there are a lot of good topics identified in this first slide deck. Based on Brad's post, it sounds like the upcoming presentations will focus on Naming Conventions, Rich Type System, and Member Types. And before I forget, if you don't currently subscribe to Brad's blog, I'd encourage you to consider doing so. His posts are always interesting and informative.
Update: Frankred's post provides a link to the Designing .NET Class Libraries page that coordinates the 14-part presentation series.