In case it's of interest to anyone, these are the current remote triggers for my S90 (of course, it'll work with any Canon). The threshold for my particular S90 seems to be about 3.7V, so I just use a 5V signal to be safe.
The first one is long range at 1000 ft, and uses Linx's long range high power 433 MHz transmitter and receiver pair. It is built from scratch as a prototype and so is very bulky right now. The relay is to switch a higher-power load, and isn't for the camera's trigger. The camera USB cable connects to the small header on the board.
The second one has a theoretical 300 foot range, and is based off of the two-way Phottix Plato (~$40). The reason this is cool is because the receiver sends a signal back to the transmitter, so the operator can tell if the receiver is in range! Extremely handy to know that the signal was received. I haven't modified the remote themselves but only added a very simple trigger addition to make it work with Canon's. The addition is just an AA battery (1.5V) to 5V boost converter, a MOSFET, and a few resistors.
