The Overground timetable from December 2012 is now online.

http://www.tfl.gov.uk/gettingaround/6391.aspx

The good news is the Clapham Junction extension will be operating. The bad news is we'll have fewer trains going to Highbury & Islington. The Crystal Palace service will still terminate at H&I but the West Croydon service will terminate at Dalston Junction. This is to free up space at H&I for the Clapham Junction service.