Whatever Microsoft ends up announcing on May 20 -- whether-- the pen is expected to play a key role.
Microsoft already supports digital ink and digitizing pens with its Surface Pro devices. According to various sources, the expected ARM-based Surface Mini will be optimized to work with a pen, as well.
But the Surface team isn't the only force inside Microsoft working on improving pen and digital ink support. In fact, there is a Cross-Group Inking team working on making pen input better across Windows, Office, Surface and Perceptive Pixel devices.
The Cross-Group team was formed in the fall of 2013, according to a Microsoft Research video published by Microsoft on May 14. (Thanks to Windows SuperSite's Paul Thurrott and Lairent F. for the link to the video and accompanying presentation about the team's efforts.)
Update (May 18): Microsoft seemingly has removed the MSR link with the video and slides.
Before a "One Microsoft Ink" strategy. By the end of 2013, the unified team was talking up its plans in executive reviews, according to the MSR presentation., there were multiple groups working on their own digital inking efforts. But in September, the various teams came together to create shared design principles. By October, they had merged their strategy documents to create
Microsoft Program Manager Tucker Hatfield mentioned in the MSR video that the Windows team is looking to incorporate the new digital-ink capabilities in Windows "Threshold," which is believed to be Windows 9, expected to arrive in the spring of 2015. As Thurrott noted on his site, the unified digital-inking technologies also seem to be on the roadmap for inclusion inthe touch-first, Metro-Style Office apps Microsoft is building for Windows, and which are expected to arrive in the fall of 2014.
It's probably too early for any of this cross-platform work to show up in the Surfaces that are announced on May 20. The concepts and suggestions in the MSR video from May 14 represent a from-the-ground-up do-over of what Microsoft has offered in the past in terms of pen/digital ink support. But it does seem a new, unified platform is in the works and may begin to show up in various products starting late this year.
A related aside: Thanks to Martin Anderson (@mdtauk), we also know a new Microsoft codename. Mentioned in the MSR video is a natural language commanding system in Office, which is codenamed "Oz."