This review for CBBC news states:

'... the Timelord is ready to be told an important message from The Face of Boe ... this is the episode where the Doctor is told a secret that will overshadow the rest of the series. Four words long, it will send shivers up your spine.'

[ http://news.bbc.co.uk/cbbcnews/hi/newsid_6530000/newsid_6539000/6539021.stm ]

However, there's a chance the episode might not be shown this week. If the football overruns, then it'll be put back to the week after.

[ http://news.bbc.co.uk/1/hi/entertainment/6544729.stm ]

The Face of Boe does say something along the lines of "YOU ARE NOT ALONE" but this may not be the exact words that he says. Its possible that the Mondas Cybermen return and that they are the creatures from beneath the fog. This has yet to be confirmed.