At this point developers have already had access to a preview build of the Chromecast SDK, but there are restrictions put in place limiting public distribution. Recent reports suggest we could be seeing some changes come to these restrictions kicking off as early as next month.
It looks as though Google has sent out invites to several developers to a Chromecast event described as a hackathon that will take place on December 7th and 8th in Mountain View. Attendees will be able to check out an "upcoming release" of the Cast SDK as well as have the chance to speak directly with Google engineers regarding "best practices," among other things.
Some of the devs invited included Thomas Kjeldsen and CyanogenMod developer Koushik Dutta. You likely remember Dutta from his work with AirCast (a Chromecast mod Google apparently shut down through a software update), according to his Google+ page he won't be "applying to go," because he is "too busy" with Cyanogen.
Either way, it will be interesting to see what the development community will come up with. In the mean time, Google has released an update to the Android Chromecast app that comes along with some time zone and Chromecast MAC address display tweaks, as well as a refreshed design to closely match the company's latest design aesthetics.