Hi
i actually tried to do this a few years ago. The threads above are correct in that the pulse oximeter is the way to go. You can get a box of them on Ebay: they have a proprietary connector that gives you access to the led and a phototransistor.
Now the bad news:
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You need some pretty sweet, very low noise operational amplifiers to detect the change in the phototransistor. If I remember correctly, a heartbeat caused a 10mv change in the photosensor's value.
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These pulse oximieters are designed largely for people who are lying in bed and not moving around much! Every time you move your arm around, the blood gets sloshed around in your arm, and influences the signal coming from the phototransistor. Add that blood-sloshing-noise to your 10mv change detector above, and you have got a pretty serious analog and digital job on your hands. The computerized circuitry that detects the light change in the commercial oximeters filters out such movement anomalies... so you will have to figure out how to do this in yours. Headache number two.
I never really got it working the way it was supposed to... but if you can, I want the schematic! 