The news broke today following the BBFC's classification of extras, which included an interview with Hartnell among them.
Doctor Who Restoration Team member Steve Roberts confirmed that it was an in-vision interview - meaning that it was on screen as opposed to just being audio - and stated that it was fellow team member Richard Bignell who had discovered it.
The interview was conducted by Roger Mills for Points West - the BBC's regional news show - and was held in the dressing room of the Gaumont Theatre in Taunton, where Hartnell was appearing in the panto Puss In Boots less than three months after last being seen in Doctor Who. The interview aired on Tuesday 17th January 1967.
The rest of the article is here, including a tidbit that David Bradley was shown the interview as part of his preperation for playing Hartnell in the 50th anniversary show biopic "An Adventure in Space and Time." It'll also be an extra in The Tenth Planet DVD release."
Pretty exciting, eh? And I can't help but wonder whether some half-understood mutant rumour about this discovery was somewhere at the bottom of some of the "lost stories" scuttlebutt flying around a month or two ago? Well, time will tell...