The HDMI Specifications do not specify cable lengths. However, for long cable runs the following is recommended:
If you need to send a 1080p signal more than 25 feet, or a 1080i signal more than 49 feet, active electronics will help clean up and boost an HDMI signal when it’s traveling long distances. You can purchase either a stand-alone booster or an active cable, where the circuitry is embedded in the connectors themselves. In both cases, the connection requires a power source.
HDMI over CAT5e/6:
Runs of up to 50 meters can be achieved by sending the HDMI signal over a run of CAT5e/CAT6 networking cable, using special adapters designed for this purpose. These products incorporate booster and equalization electronics at each end of the path and require external power. Because of its tighter manufacturing tolerances, CAT6 cable is generally preferred over CAT5 in these applications.
For more info on HDMI see: http://en.wikipedia.org/wiki/HDMI