Briefly: Microsoft has a "small gathering" for its Surface tablets planned for New York City on May 20th -- read between the lines, and it looks like a reduced-size Surface is on the way. An 8-inch tablet would make the Surface even more portable, and give the reigning champion iPad Mini even more competition.
A mini Surface would need a new range of keyboard and case accessories; Neowin suggests that the new device would include support for a digitiser pen. Apparently Microsoft wants the Mini to be a good note-taking tablet, despite that range of screen sizes generally being used for content consumption rather than creation.
Further credibility to the rumour comes from the fact that Vostrostone, an electronics accessories manufacturer, has a case listed on Amazon for a Surface Mini tablet. The picture associated with that listing is a Lenovo Thinkpad Tablet 8, but the description specifically mentions its compatibility with an 8-inch Mini.
All the hints point to a new, smaller Surface Mini being launched in New York in just under a fortnight. It's not yet clear whether the Mini will run full-fat Windows 8.1 (Update 1, naturally), or whether it'll be one of the last Windows RT holdouts.