I had the same problem with my 3 week old Westinhouse TX42F430S - none of the 4 HDMI ports would detect an input signal.
The quick fix for me was to "reset" the TV by doing the following:
1) turn it off using the side power button.
2) press and HOLD the side power button for at least 10 seconds, then release.
If you've done it correctly the TV should turn on and display the white "W" Westinghouse logo, indicating that it is performing a full system reset.
This seems to have permanently corrected the problem for me. I guess the previous poster's fix of unplugging everything and waiting a minute (some say an hour) essentially causes the same "reset" procedure to occur, but I think the 10 second power button trick is much easier.
The explanation I read on another forum was related to the HDCP copy protection algorithms used by the HDMI ports. Apparently the source devices (cable box, Blu ray player, etc) perform a protocol "handshake" with the TV's HDMI ports to exchange encryption keys before they can transmit a signal to the TV. It seems that occasionally the TV's HDMI "keystore" can get corrupted and the source device refuses to send a signal (hence the "no input signal detected" message on the TV) On the TX42F430S all four HDMI ports share the same single HDMI circuitry (they use a "hub" to provide 4 ports) so a corrupted keystore will effect all 4 ports at once.
By doing the 10-second power button reset (or unplugging everything and waiting some amount of time) the HDCP keystore cache is cleared and the devices are able to renegotiate the crypo keys.
Alas, gone are the days when you could just plug in a cable and have it work.