Windows 7 , Nvidia 580 + Nvidia Titan


2.68 , Worked in 2.62 at least maybe even in more recent ones.

When rendering in blender internal you could select RGBA for output.
This would give you a image of just your object , on a alpha channel, No background.
In version 2.68 the gray background seems to persist no matter what you do.
I tried the same thing on a other system with a old version 2.62 to check if it was not my mistake, but it worked there.

It seems that by adding the blender internal viewport render ( which is nice by the way ) some other things got broken.



Select RGBA for output, and check if the grey background is replaced by a alpha channel.