I have an Arduino duemilanove that is wired to a homemade LED matrix that occupies all the digital pins (plus two of the analog). I have also wired in two LM35 thermometers, both sharing a line to the 5v power pin, sharing a second line to the ground, and outputting to two different analog pins. The entire setup is currently powered by USB.
Now, when I only had one thermometer wired in, it worked perfectly - all the lights worked, and the thermometer read values accurately. However, after I wire in the second thermometer in that series, it works for about ten seconds, then all the lights on the arduino itself slowly fade out until only the power LED is just barely lit.
Is the 5v pin not strong enough to run both thermometers, or is it because that wiring scheme is faulty? The software never runs more than one LED at a time, so I would be surprised if the board was not able to run two LM35s and a single LED simultaneously....