There's still a lot we don't know about the Surface Mini, including pricing, distribution plans and even whether the tablet will be powered by an Intel or ARM processor. However, original leaks about the Surface Mini pegged it as an ARM-based device running Windows RT.
Update (May 6): Neowin and Windows SuperSite are corroborating that the Surface Mini will be an ARM-based device with real, digitizing stylus. OneNote will be key to the platform, given it is expected to debut as a notetaking-centric device. The eight-inch device will come with covers that will also function as kickstands, according to both sites. Both sites are reporting the Mini will likely be available in June (probably the latter half of the month).
Windows SuperSite is also claiming there could possibly be a second Surface device of some kind announced at the May 20 event, but no hints as to what that device may be.
Update (May 7): Bloomberg also is reporting there will be more than just an ARM-based Surface Mini. One or more Intel-based Surface devices may also be unveiled at the May 20 event, Bloomberg said. It's not clear if these will be smaller, Atom-based Surface Minis or larger-screen successors to the Surface Pro 2.