It certainly can make a difference, and that difference might be quite visible.
Agreed!!! I've got Samsung slimline 40'' LCD hooked up to a Sky HD box and a Sony Blu ray player. I've had picture and sound 'failures' because my cables were next to the 240v leads. I've re-routed them away from the HDMI cables and gone to the '1.3 cable' which is around £10 for a 2 metre length. The difference is really noticable.
This article will help illustrate:
An introduction to HDMI cables - AVReview Features