If its a Dazzle or similar capture device it doesn't void any warranties as all it does is take your analog output, convert it to digital via the 1394 connection and you simply save it to an avi file on your hard drive. You can use any similar technology to achieve the same result, including those multimedia computers that have video analog input jacks.
What the OP may have been thinking of is a similar system that permits capturing HDTV. This can also be done using the analog VGA and audio connections to a high end encoder (not cheap) What can't be done is HDTV via the DVI or HDMI as these are all encrypted HDCP.