PC-world has a look at cables.

http://pcworld.about.com/magazine/2309p111id121777.htm

They're testing both HDMI and Component cables, from cheap to expensive.
Conclusion:

The Big Picture

Whether you hook up your TV via digital connections, analog connections, or both, you are unlikely to detect any difference in picture quality between a cable with a moderate price and a luxury brand. The only difference you're likely to notice is how the cable looks behind your TV.

The test:

http://pcworld.about.com/magazine/2309p111id121777.htm

Hi Soundproof,

I agree entirely!  Digitally encoded signals at low to medium frequencies seem to cope with almost any type of signal path without degradation.  HDMI cables just need good connectors to ensure reliable coupling and they should be fine.

However, with audio cables, I would argue that's a different matter.....

Best regards,

Steve.