This is not a bug, but changed behavior. OpenGL render now follows Alpha Mode from Shading panel, meaning if you'll want opengl render with sky applied, no transparency will present (sky could not be transparent). If you want OpenGL renders be transparent, set alpha mode to Transparent.

I've updated release notes there: http://wiki.blender.org/index.php/Dev:Ref/Release_Notes/2.66/Image_Transparency#OpenGL_Render_Alpha

Thanks for the report, but it was intentional change.

There is no Shading panel in Cycles render!
And there is no any option in Blender Game render also.

Maybe it's better to place this option somewhere in World settings, make scene to me.

This is known issue and would be addressed for 2.66. For now some more critical crashes are here to be solved.

Meanwhile you could switch to BI to change alpha mode used for OpenGL render.

I apologize if this is posted in the wrong place. This *seems* like a bug. Using Blender 2.78.4 which I used instead of 2.78a/b because prior to 2.78c IK was broken and causing crashes. i will test on 2.78c asap.

I am using png's mapped to planes, the idea being to use opengl renders so that mapped geometry can be render fast with Grease Pencil art / animation. As seen here in the two images, if you have two planes that overlap. As long as a plane is SELECTED, the viewport opengl handles alpha as expected, but as soon as you UN select everything, there is a visible halo which I presume relates to how blender is handling premultiplied alpha (?)

png, tiff, tga, psd, does not matter, same behavior with all file types. Different display settings such as "Only Render" and "World Background" also make no difference. Carefully preserving the aspect ratio of the imported art ( in case scaling the geometry was introducing issues ) - no change. High subdivision levels on the geometry, None of these help. I even tried multiple planes in 3D space at different Z levels to see if perhaps by selecting the one closest to the camera eliminated the issue bc it is confusing since the issue disappears when a plane is selected.

opengl render seems to be unusable b/c if you unselect everything the alpha channel halos show, and if you select something it has the "selected" indicator. If you turn off the "selected" indicator as a kludge, it *sort of* helps IF there are only two planes overlapping. If you overlap THREE, whichever one is selected has a clean alpha and the two that are not selected have the gruesome halo lol.

behavior is identical on 2.78c