I have seen a number of questions, rumors, and experiences posted about HDMI cables, so I thought it might be useful to give some comments to the community to help clear the air.
First off, the vast majority of image quality or interoperability issues with HDMI devices are related to the software used for device communication and content protection, and have nothing to do with the HDMI cable. In particular, these issues are often caused by the software related to HDCP handshaking, or from devices improperly handling the device capability information read through HDMI (e.g. the device has an incorrect EDID, or an inability to properly read an EDID). It is fairly uncommon for the cable to be the cause of HDMI compatibility problems, or for non-compliant cables to be found in the market. In fact, the robustness of the HDMI specification has been verified by the fact that we have not found a compliant HDMI cable that is the root cause of HDMI playback issues with compliant devices.
All HDMI cables are required to support, at minimum, a standard HDTV video signal (i.e. 720p or 1080i) by virtue of being tested to verify that they meet the HDMI spec requirements. This is referred to as a Category 1 test. More recently, the HDMI Authorized Testing Centers (ATCs) have added equipment to be able to test the cable's ability to support 1080p (which is 2x the 720p/1080i video rates) and higher rates up to the maximum HDMI speeds. These higher speeds are called Category 2. Since 1080p and deep color are becoming more common market requirements, we are seeing cable manufacturers wanting to have their cables verified with this Category 2 high speed test instead of the Category 1 test so that they can market their cables as being 1080p verified. Simplay Labs is another HDMI testing service that has been performing this high speed cable test for over a year, and some cable makers are putting the Simplay HD logo on their HDMI cables as a way to convey this level of quality.
Note, however, that cables that were Category 1 tested may still fully support and be capable of passing the Category 2 test. As a general rule of thumb, we have seen that shorter HDMI cables (i.e. 3m or shorter), even those without a specific 1080p marking, will likely pass the Category 2 test. As the length goes much higher (such as 5m or longer), the high speed 1080p signal becomes more demanding on the cable's quality.
It is most important to note that the quality of the HDMI receiver chip (in the TV, for example) has a large effect on the ability to cleanly recover and display the HDMI signal. A significant majority, perhaps all, of the HDMI TVs and projectors that support 1080p on the HDMI inputs are designed with quality receiver chips that will cleanly recover the 1080p HDMI signal using almost any typical length cable, including those that only passed the Category 1 test. Even though the cable is not officially guaranteed to support these higher speeds, the reality is that it usually does work fine. This is good news for consumers, especially those that have already purchased HDMI cables before the Category 2 testing was widely available. It is likely that even the lower cost cables will work fine, even at high-speeds, so long as they are tested to the minimum (Category 1) requirements.
Personally, I have seen demonstrations by HDMI semiconductor companies showing a 1080p signal run on a 50 ft cable with a clean image, and a 720p signal run on a >75ft cable also with a clean image. It's likely that these cables would not even pass the Category 2 cable test, but this seems to indicate that a high quality receiver chip (particularly those with equalizer electronics built in) can have a significant impact on the signal margins, and thus the ability to have clean image with a lower quality and/or long cable.