I thought this looked interesting over at the inquirer.
Plugging into a PCI express x1 slot (finally - a use for it!), the $250 Intensity allows capture of HDMI streams at up to 1080i or 720p (but not 1080p, a little disappointingly), and works on either Windows or Mac OS systems. Also, being HDMI, it can capture video and audio simutaneously, without any need for assistance from a separate sound card, which is helpful.