After the public availability of the SharePoint 15 CTP, now we have a new announcement SharePoint 15 SDK CTP has been released.
It's intended to provide an overview of the future SharePoint 15 object model in order to take in account on future migrations or on new developments.
Btw, i would like to remind you:
Community Technical Preview (CTP) builds do not go through the same rigorous testing that beta builds undergo. While betas receive a much higher level of testing and feature work, CTPs are intended to expose developers to the latest working build. CTPs are therefore unsupported, prerelease software and there are some precautions you should take. Please read the latest Readme and Installation Guides before you download or install prerelease software.
Héctor Calvarro Martín. SharePoint Dev. Microsoft Support Engineer for DEV EMEA