Re: Cat6 as HDMI?
If you want to pass a 1080P signal [twice the bandwidth of 1080i] over CAT5, the losses in bandwidth may make this unworkable, especially over 50foot; thus CAT6 is reliable for HDMI runs around 100 feet. However, if this in done in a commercial environment, or the cables pass by high power A/C units, the twisted-pair shielding is not enough; there is CAT6 shielded cable made for this. The baluns are not cheap at $1000/pair [Muxlabs] but they are bullet proof. This is from hands-on experience I have encountered, working with Custom Install for a large AV retailer.
For the home, a pre-made Audioquest/Monster/etc cable of 40' plus will be ultra-reliable.
As is always true in A/V, you get what you pay for.