I posted an introduction to Polymer a while back, here is the follow up post with more info on how it is licensed, the community and where to go to get involved.
Currently, Polymer implements a set of polyfills to make current browsers compatible. Eventually the polyfills will be eliminated as browsers get native support.
The code is licensed under BSD. You can modify or distribute the code as long as the copyright information is included, the disclaimer message is provided and the names of its contributors are not used to endorse or promote.
You can get the code here and find documentation here. The Polymer developers hang out on IRC in the #polymer freenode channel. To get in the loop join the Google Groups mail list. For just the highlights you can follow them on Google+ or Twitter.