On the Windows version Adobe is accelerating H.264 with UVD2 for ATI Radeon graphics hardware, NVIDIA hardware with PureVideo, Intel GMA 4-Series acceleration support, and even Broadcom video acceleration support via their Crystal HD Enhanced Video Accelerator. Adobe didn't provide any Linux video acceleration support at this time because Linux "lacks a developed standard API that supports H.264 hardware video decoding."
NVIDIA's VDPAU is rather developed on Linux and well adopted and does a rather grand job at accelerating a variety of different formats. Previously it also looked like Adobe was going to use VDPAU. There is also VA-API as another nice alternative or as a secondary choice. Hopefully Adobe engineers will bring VDPAU and/or VA-API support to a later Flash Player release. The free software Gnash Flash Player even supports VA-API with patches.
Besides the H.264 video acceleration work, Flash Player 10.1 features the first runtime release of the Open Screen Project, mobility optimizations (should be more CPU and battery efficient), accelerometer support, and new features for Flash/SWF developers too.
The latest Adobe Flash Player 10.1 Beta release can be found at Adobe Labs.
There is also the first beta release for Adobe AIR 2, which includes 32-bit Linux support but the 64-bit support is held up by waiting on a 64-bit Flash Player 10.1 build. Details and downloads for Adobe AIR 2 Beta can be found at this Adobe blog entry.