Several of my clients have older MacMini's set up in conference rooms or as lobby displays (rotating company bio videos) or as media servers in break rooms. Playing 1080p has been troublesome and I've had them ask a few times why don't I produce things in "full HD" for them until I show them the hardware trying to play it smoothly, but apparently Broadcom's Crystal HD Hardware Decoder enables hardware decoding of 1080p content...but only with XMBC.
Through hard work and the joint efforts of several TeamXBMC/Redhat developers and the Broadcom Media PC Group, cross-platform hardware decoding of mpeg2, h.264 and VC1 video content up to 1080p will be coming to XBMC on OSX, Linux, and Windows via the Broadcom Crystal HD Hardware Decoder (BCM970012).
The Broadcom Crystal HD is available now in a mini-PCIE card with ExpressCard and 1X PCIE form factors to follow. This means that the AppleTV and all those lovely new netbooks, Eee Boxes and older Intel Mac Minis have exciting new potential.
Looks like it's about $69. The model number is BCM970012.
Be aware that OSX drivers for the card are pretty green as of this post so Use At Your Own Risk.