I had two Arduinos (one Diecimila 328, one Pro 328 5V) connected to Parallax serial-interface 4x20 LCD units (Parallax #27979), and they worked for several months of continuous display, then failed. I was driving the displays using NewSoftSerial and pin 7 of the Arduinos.
I loaded the Arduinos with what was effectively an all-pin version of the Blinky program. With this tool and a scope, I found that on both Arduinos, every pin could put out a nice, more-or-less rail-to-rail square wave, except that
- pins 5 and 7 of the Diecimila are essentially dead- pins 7 and 8 of the Pro put out square waves that peak at +5V but bottom at +3.12V and 2.24V, respectively
I assume that I tried to draw too much current from the pin 7s, and that I've burned through some pull-down resistors in the chip. I wouldn't expect the Parallax boards to draw very much power on their RX lines, and the coupled failures of the non-7 pins is strange, but I have no other theory.
It's easy enough to make up a workaround with a transistor, a resistor, and ( :() pin reassignment or ( 
:() replacement Arduinos, but I wonder if I'm missing something. Have other people had pertinent experience with the Parallax interfaces?