There are a number of rumours as to what it could be, with Heavy Rain being heavily spoken about by most PS3 users who have read the above article.
As posted on NTHW, there are currently 20 titles in top secret development.
Other than Heavy Rain, The Getaway 3 has been heavily tipped as being this top secret title.
Other news suggest Motorstorm 2 and Resistance:FoM 2 out this Christmas.
There is currently a book running on it at NTHW!
Guess we will just have to wait and see.