Holy crap, Microsoft! When did you get relevant again? This is exactly what we creatives want. Apple's been halfheartedly playing in mobile way too damned long ... about time they got a sucker-punch like this.
Later: Anandtech has a bit more info.
Even later: Reactions among the designer community. Predictable, but still worthy.
Even later later: DP Review: "A number of apps at launch, like Paint 3D and Sketchable, will support enhanced capabilities with the dial and pen, and we expect creative applications, like Adobe's Creative Suite, to quickly follow suit, particularly because Microsoft has made it easy for developers to do so."