So! There has been many a lengthy discussion about the story arc and whatnot, and a few things I've noticed be these:
Seen as a notice on a Taxi.
Phil Collinson: Theres something about that Taxi that you should keep you eye out for.
Tennant: The art department are able to start laying a few little hints for something thats coming up later in the series.
Summery for The Sontaran Stratagem:
Having covertly established a base at the mysterious ATMOS Factory aided by the Rattigan Academy, the Sontarans unleash a potent terror which brings the world to its knees.
Atmos is also Greek for "vapor", leading people to link it to atmos-phere = The Poison Sky
I'm not as sure about this one, but does that drawing on the wall look a tad Dalek-like?
What looked like the word/initials DUST appeared in the episode behind Miss Foster during her presentation, although its now believed that this was just part of the word "inDUSTries"
During a conversation with the Doctor on the rooftops, Donna mentioned that all the Bees are apparently disappearing. It may be nothing but because the Doctor follows it up and questions it, it might be more significant than we think...
And then of course theres this:
There we go! Let the speculating commence... err... further!