I have a specific fix that worked for me. Fresh install of Windows 7, graphics drivers (nVidia) installed. Tuscany Rift demos show a "blue rectangle" - and all other Unity-based demos showing a similar uniformly coloured rectangle (probably the selected colour to clear the colour buffer with in each case, in other demos this has been grey or black).
Installing this specific package caused all stereo Unity demos to work immediately after:
Microsoft Visual C++ 2010 x86 Redistributable.
(Make sure you grab the x86 one)
Knowing this would have saved me some time/pain
(I don't think it's in the SDK doc for setup, at least 0.1.5 anyway. Clearly you wouldn't need the redistributable package if you already have visual studio 2010 and are compiling/running C++ apps on the machine, but you do for deployment on other machines. Anyway, figuring it out was non-trivial for me since the Unity apps did run and there was no complaint about missing DLL, etc., which would clearly indicate the problem)
Anyway hope this helps someone!