that behavior went away using the 275.33 driver
so i think this:
on Windows the latest NVIDIA GeForce driver 280.26 causes problems with the first initialization of the alpha buffer, which we couldn't work around yet. for the time being please switch back to the previous stable driver release 275.33.
might still apply for this version, if not for the alpha buffer, then at least for opacity.
interestingly this wasn't the case in the OctaneRender® 1.025 beta 2.51 pre-release, where everything worked fine for me with the 280.26 driver version.
also the responsiveness seems a tad worse with the 275.33 driver than it was with the pre-release and version 280.26.