In the past I have settled for tapping one of the AC wires, filtering it a little, and letting my datalogger (PICaxe) count pulses. This isn't very reliable, on my WT generator, because there seems to be a lot of harmonics in the AC. I operate a permanent-magnet motor conversion, so the harmonics are exaggerated with all that iron in there. I expect an axial PMA with air-core stator has much less of this noise and in that case the AC will work better than in my experience.
Hugh Piggott probably still has a good diagram of a tachometer circuit on his Scoraig Wind website. I have built it and it does work, but I made some mistakes, so I do not have it working any more reliably than the straight AC tap that I mentioned before.
I've use the bike speedo before, too, but only when doing bench-tests of generators before putting them on the tower. The speedo gives a good reading but obviously you can't record it without a pencil...
What I really should do it put the same kind of magnet/reed-switch combination on the actual wind turbine, permanently, with wires coming down the tower to read the signal from the reed switch. No AC harmonics and very low power requirement. How come I never get around to doing simple things like this, but waste days building tachometer circuit boards instead?