MarkT:
Sounds like the opto coupler is either broken or wired the wrong way round - you're giving it a good 30mA or so of drive - have you checked that each side of the opto coupler is connected the right way round?And those values aren't very meaningful without all the code - or some idea of what you are expecting (whats generating the pulses? how many per revolution?)
it says it all in the post, 12V dc motor from pc fan running at 9V set up to block the photo-interrupter every revolution. Not sure how fast it is suppose to spin but the values am getting are way too random. I am sure the photo-interrupter works because the arduino detects when it is blocked. The code am using is from the readingRPM page from arduino playground.