Well as I told lefty, another discussion in a different section about possible shield ideas lead me to consider adding LED status circuits to each of the digital lines on my Cidekick project http://arduino.cc/forum/index.php/topic,67044.msg493322.html#msg493322
but I was trying to figure out how an isolation circuit would work for each pin or if it was even needed.
The idea was an LED shield that would show the status of each pin high or low and I was thinking RGB for the pwm pins rather than just fade.
The whole idea would only work if it it could be electronically transparent, meaning not affecting other circuits or uses to display the status and if it was low component count.
Yeah, its a nice idea - I thought about this and the best I could think of was using a cheap microcontroller with enough inputs (ATmega 168?) and a bargraph style LED array wired in Charlieplex style. So long as the update is fast enough the human eye won't notice... And having a micro controller means being able to do some of the functions of a logic analyser. You'd not do anything with the analog pins (and the 168's analog pins can be used as digital out to drive the charlieplex array.
The Arduino's digital pins only see one more CMOS input load, not going to affect function.
Couldn't think of a clever way to spot floating pins though, that would be useful...