Tech Report tested it and claims they've noticed some improvements while watching YouTube videos:
Tom Barclay, senior product marketing manager, Platform Business Unit, Adobe Systems, said Adobe Flash Player 10 delivers enhancements and new features such as new support for custom filters and effects, native 3-D transformation and animation, advanced audio processing, and GPU hardware acceleration. In addition, the new release builds on Adobe's expertise with text to deliver a new text engine that provides interactive designers and developers with more text layout options and better creative control.You can update your Flash Player at Adobe.
We gave the new plug-in a shot, and while we didn't see a huge difference in CPU usage on a Core 2 Duo E6400, maximizing YouTube videos did feel noticeably snappier. Disabling hardware acceleration through the plug-in settings somehow didn't affect that, however. After investigating the matter, we found this blog post written on May 16 by Adobe software engineer Tinic Uro. In the post, Uro says web developers actually have to enable GPU acceleration manually in the code. Stranger yet, the new GPU-accelerated modes can supposedly slow down content, because "the software rasterizer in the Flash Player can optimize a lot of cases the GPU cannot optimize." So much for that.