Bill, you purchase a system from a source, and if it is made correctly it works. The units that have a stronger magnet generating the power pulse for the remote sensor transmitter can indeed have the whole thing set in epoxy, since there would be no battery to replace.
Unlike TV remotes, I would prefer that the communications not be universal, in order to reduce the value of the pieces to would-be thieves. That is a problem here, where even a plain bike will wander if it is not securely locked up.
The sensor that I like best uses 2 magnets at 180 degrees on the rear rim, and the sense pickup bolted to the frame, so that it is out of the way and out of harms way. Not as light as some, but that has never been a problem for me. A simple charging circuit and after a short distance the CMOS counter would display speed fairly well. And it was not really wireless.
ANT+ Sport is quickly becoming the de facto standard out there.
Garmin, PowerTap, EnkiSports, DigiFit, Tanita (scales) and others use the chip/comm standard; meanwhile, I've heard Polar uses the chip with a proprietary comm scheme.
More info about the 2.4GHz ANT+ Transmitter:
Disclaimer - I don't work for, nor am I affiliated with ANT+. I'm an athlete and engineer who has been using the products for years and hope to see it in phones soon. Luckily TI is working with them to do just this: http://bit.ly/bVkRyD