Wow, how did I accidentally drag this back to life? Occasionally I dredge up an old thread in search results and don't look at the date before answering the question, but I haven't done that lately. Hopefully the stuff here will be helpful to someone in the future.
As for reed switches vs hall effect sensors, I don't think it's correct to say that all of the magnetic ones use reed switches, because hall effect sensor is just a solid state reed switch. I'm pretty sure my Shimano computer is a hall effect sensor. The ANT Sport sensors I've used (which look identical to the Trek sensors) are reed switches because I can hear them tick.