We have not published any particular "UI Specifications", because we don't feel it's our business to tell you what your widget should ook like, especialy if you alrady have a particular look and feel and branding in which you already have some investment.
For instance, our Digg widget looks like Digg's site, our Surfline widget looks like Surfline's site.
However, there are some pretty basic rules that are dictated by the form factor - for instance, it's pointless to make really small buttons since people will be using their finger to navigate, hence buttons should be around 1/2" square or thereabouts.
We have published some sample widgets, and are in the process of preparing more that will be the basis for new widgets and replacing some of our older widgets - they'll show how *we* think things like RSS readers should respond to use actions, including the accelerometer, and can be used as the basis for similar third-party widgets.
There is no "dev kit" for chumby - all you need is Flash and the documentation and examples on the wiki.
The light sensor has been removed for the production units, so there's no real point in coding for it going forward. We found the sensor to be too imprecise at low light levels to be useful, and decided to replace it with a microphone.
Similiarly, the bend sensor has been replaced by a switch. The bend sensor was too imprecise, tended to be damaged by overethusiatic squeezing, and was available from only a single source.
The author information is stored on the server - the production chumbys will allow the user to view the author's username, and the site will be modified to give better attribution and access to more information about individual authors and their widgets. We're also adding a ratings system.
The Google Calendar widget is simply an RSS widget using a feed published by Google - it's a close cousin to the example widget posted on the wiki.
The Control Panel is begin completely rewritten and is a moving target at the moment, so no documentation for now.