There is not ATG Seibel integrators or framework available as of now. Oracle is in the process of creating a framework as part of the product stack - but the times lines are not available.
That being said - the standard way to integrate is to use webservices / soap based communication in combination of batch jobs or processes. The selection of batch jobs vs webservices to synch data between ATG and Siebel is totally an architect decisions based on your business need.
The product development team is working on a reference integration between ATG and Siebel. This project has been broken down into three phases. The first phase was the account profile integration. Which is available right now on the support site as mentioned by Srinivas. We are currently working on the second phase which consists of product data sync, shopping cart and configurator. Once that is done, we will start to work on Phase 3 which consists of eligibility-compatibility, ABO, dynamic pricing and checkout.
If anyone needs more information right now, please feel free to contact your sales rep.