At first I thought I'd do a "half-duplex" solution with a single fan being both the anemometer and the powered fan. Unfortunately, it takes a lot of wind to blow a case fan around. I discovered that if you take the coils out of a fan, it spins much easier! Which lead me to a "full-duplex" solution of one gutted fan as an anemometer, and another as the actual fan.
To measure the anemometer, I started out with an NTE3100 slot-type opto-interruptor, using a piece of aluminum from a can to slide through the slot:
This had the advantage that the NTE3100 available at Fry's. The disadvantage was that the aluminum bit takes up excess space and could get bent.
We all know that Radio Shack isn't what it used to be, but guess what, they still carry 276-142 "Matched IR Emitter and Phototransistor". So I mocked them up on a breadboard with the Arduino to ensure it was dependably reading over the distance required:
Next step: I need to get another NTE2987 (N-ch, logic level FET) from Fry's. I also need wire wrap sockets from Fry's so that I can stick an Arduino ProtoShield in between the Arduino and the Ethernet Shield. Then just solder it all together and do the software!
Also I may move up from a gutted fan to a real anemometer impeller (with jewel bearings, etc.)