The new Flash Player for Mac release, version 10.1.82.76, enables the hardware acceleration of H264 on a number of Macintosh computers. The feature has been in beta for a while as Gala, and, in a somewhat uncommon course of action, Adobe decided to include the feature as part of a regular security release.
The problem is that Apple's support for this new framework is sketchy. Only Macs equipped with NVIDIA GeForce 9400M, GeForce 320M or GeForce GT 330M GPUs are supported, so anything by ATI is out of luck (the current generation iMac and Mac Pro, for instance).
Even if you do have a Mac with any of these three GPUs, you might still run into problems as there are a number of driver limitations. For instance, "video clips with a width of 864 will fallback to software H264 decoding on the nVidia GeForce 9400M. This is a known limitation of the GeForce 9400M drivers," Adobe writes, "Unfortunately, YouTube Clips at 480p resolution often use a width of 864. Switch to 720p or 1080p to benefit from hardware acceleration for these clips."
Get it while it's hot.