The license we use for the SDK is LGPL-3.
The only thing you should do is to be compliant with this license.
What it really means in terms of actions, I'm not sure.
Unless it changed, I believe a linking
exception was added to the LGPL-3 used by CB's SDK; but, I have NOT really read it since it changed from L/GPL2 to version 3.
If still true, you need to make available all the source code changes made to CB's SDK; but, you need NOT make your plugin source code available if it just dynamically links to the SDK. But, some complex plugins do need to edit the SDK source code to work; you will need to make available those changes and you need to make available all of the source code from the CB project that you plan to distribute.
NOTE: The art work used by CB might be under a separate license from LGPL-3.
Edit:
http://www.gnu.org/licenses/lgpl-3.0.html; I did NOT see the old linking exception in the SDK source code; but, I think LGPL allows the linking exception, the words were too confusing for me to be sure that is true.
Tim S.