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The Official Doctor Who site have just confirmed that a celebratory trailer will air at 20:20 on BBC One (HD) just before Atlantis. The press release reads as follows:

A specially created trailer celebrating the last 50 years of Doctor Who will air tonight on BBC One, as an exclusive image is revealed today featuring the 11 Doctors.
Travelling through time fans will be taken on a journey from the very beginning using state of the art technology.  The special trailer is set to show all of the Doctors as they first appeared on screen, including William Hartnell in high res colour for the very first time, as celebrations ramp up to the 23 November.
A huge moment for the BBC, the 50th celebrations will culminate with the special episode, ‘The Day of the Doctor’, starring Matt Smith, David Tennant and Jenna Coleman with Billie Piper and John Hurt.  A whole range of shows have also been commissioned across TV and radio to mark the anniversary

It is expected to be online shortly afterwards. Please note, it is not a trailer for the actual 50th Anniversary special.