I thought I'd link to this bit of meta here because I came across it on the
doctorwho comm and thought it was kind of excellent, and it does relate directly to the 50th anniversary. I'm not sure whether I completely buy this theory, which relates to the character of Clara as featured in the recent Christmas special The Snowmen, but I really like it and what it implies:
SPOILERS FOR THE DOCTOR WHO 2012 XMAS SPECIAL "THE SNOWMEN"!