3 figure out the interface required to interface with my tachometer signal or a single spark plug impulse (manual transmission, so I definitely don't want a speedometer connection!)
4 write or obtain arduino software to recognize current frequency of tachometer impulses.
Bob, in the last couple of days I've been thinking about how to put together a robust datalogger for my car, capturing several engine parameters and interfacing with a GPS for position. Applications include (aside from generally geeking out) post track-day performance analysis, possibly lap timing (depending on the resolution of the GPS) and general position logging so I can go back and find those roads I loved on my last random drive through the countryside. Yesterday, I discovered the ELM323 OBD (ISO) to RS232 Interpreter. If your car was built after 1996 (I think), it's got a standardized on-board diagnostics interface which will tell you things such as throttle position, engine speed, and vehicle speed. It might save you some unnecessary hacking to pull back that data. I've bookmarked some OBD2 links on del.icio.us.
Good luck!
Brian