There is such a gizmo. It's Blackmagic Design's US$199 . This card is compatible with both Windows and Intel Macs with PCI Express card slots--meaning a Mac Pro. In addition to being able to capture DSLR menus, pictures, and videos from cameras with an HDMI port, it can import uncompressed video from HDV camcorders. And it captures not only digital video via its HDMI input, but also analog component video and S-video. Supported HD formats include 1080i50, 1080i59.94, 720p50 and 720p59.94.

If the Mac Pro requirement wasn't enough to put you off, bear in mind that in order to capture this uncompressed video you're going to need a fast disk array--uncompressed HD video consumes massive amounts of storage and you need storage fast and capacious enough to deal with it. Also, this card won't capture from devices that use HDCP (High-Bandwidth Digital Content Protection), so you can forget about using it with a Blu-Ray player or HD set-top box.