All of which suggests that the device may be palm or wrist-mounted. Since the event is developer-only, until our on-the-ground source can see the prototype for themselves further details (aside from those found on Twitter) are going to be hard to come by.
There's some other news, too. Valve apparently wants its Lighthouse tracking system for the controllers to be as commonplace as WiFi, where anyone and everyone can develop for and use it. Currently there over over 300 different companies using the tech for motion tracking. New Lighthouse base stations may arrive next year as well. We'll update this post with more info as it arrives.
Update: Pictures of the gizmo are surfacing on Twitter and as you might expect it looks very prototype-y.