Orac, you could look at the tach input circuitry used in the open-source "Megasquirt" DIY engine controller.
The V2.2 schematic is here:http://www.bgsoflex.com/v22/megasquirt_ShemV2.2.pdf
Look on the second page for the Ignition Coil interface, which uses a 4N25 optoisolator and toggles a microcontroller input pin when the coil is grounded.
Note that parts labeled "Wing", "John", and "Ed" refer to the names of users who suggested adding those components. There's a good discussion of the circuit options on this page:http://www.bgsoflex.com/v22/msv22.html
There's a more recent version of the board with a lot more options, including a true zero-point crossing detector for use with variable-reluctance, Hall Effect, HEI, and EDIS systems. Circuit diagrams and discussion here:http://www.megamanual.com/ms2/pcb.htm
Hope this helps. Good luck with the project!