Don't think there is per se a way to enforce HQ playback in every scenario, since the new fangled stuff in CS6 uses hardware acceleration and that is solely dependent on your graphics card's abilities how much it can handle in realtime. You might get away with tweaking the options and enabling the Allow Frame Skipping options in the Timeline panel, but beyond that you would have to fiddle with your graphics driver or even get a better graphics card/ system.
Are there any articles or help topics about this new timeline playback change from Adobe? I can't seem to find any specific help topics that reference this change.
I agree, this is a big problem. I believe previous versions were able to load the frames into RAM at full quality, and play them back that way. Hard to believe they've taken that away.
Regarding the lower-resolution playback of hi-resolution files, we've heard from a few folks that they'd rather have the hi-res over the performance increase it provided.
There is currently no way in CS6 to view the play-back at high-res.
We're looking into the options.
In the meantime, I've requested a Technical Document be created so this information can be found easily on an on-line search.
Thanks for the feedback,