One of the sources that told me about the SLT camera prototype with square sensor send me some info. Please be aware that this is a camera prototype and that there is no way to be sure yet that it is coming for real.
1) 37 Megapixel
2) 36mm x 36mm square sensor
3) Works with all current Alpha mount lenses
4) It is the biggest camera to date (beats the Sony A900 in terms of size)
5) The SLT technology permits the camera to be smaller than it would have been with the classical mirror box and optical viewfinder technology
6) New generation of square electronic viewfinder
7) 3×3 inch OLED display
The 36mm square format allows you to choose between different formats on the fly or in post production.
P.S.: Kodak once made a digital back with square sensor. The Kodak DCS Pro Back 645 (here on eBay).
SR1=probably fake rumor
SR2=rumor from unknown sources
SR3=50% chance it is correct
SR4=rumor from known sources
SR5=almost certainly correct!