From the release notes:
"Boxee can now accelerate decoding of H.264 videos using compatible graphics processing units (GPU). This means that machines with slower CPUs (such as Intel Atom) can now play high-definition content! This functionality is only available on Windows Vista and Windows 7 and a compatible video card."
Go here for a list of supported cards:
As a Linux user, I'm out of luck for vdpau acceleration at this point. I get mostly smooth 720p playback, but as I'm using the ION nettop, any multitasking at all will rapidly degrade performance. Hopefully this will be fixed soon as it was part of the alpha.
It also looks like WinXP is out of the loop also. And the list of supported video cards is short. I'm sure this will all be beefed up soon...
Boxee BETA (always the latest)
HTPC - ASRock micro-ATX w/ NVidia 8200 onboard graphics (VDPAU) (AMD Athlon2 X2 2.9Ghz dual-core, 2GB RAM, gigabit LAN), bluetooth mouse/keyboard combo, Microsoft MCE IR remote
Linux Mint 9 Isadora 32-bit (Ubuntu 10.04), VGA-out, Optical Audio-out
Home-Server - Old-ass Athlon single-core system w/ 512MB RAM, gigabit LAN, 1TB external HD
Ubuntu 9.10 32-bit server edition, SABnazd, Episode Butler, MediaTomb, Samba, NFS