Not that I'm looking to fire multiple cameras but I'm in the market for a wireless remote and found the following. I think many can do multiple cameras, you would just need multiple receivers - always good to confirm however.
PocketWizard at http://www.pocketwizard.com …ls/remote_camera_trigger/ can control multiple cameras. Nice thing is what they call an "A" zone that essentially keeps the camera awake (at the expense of battery life). I realize you can also address the issue of the camera going to sleep through its own menu.
Also, Popular Photography just recently recommended the Satechi Wireless Remote ($60 for Canon). You can find it on Amazon. Among other things it can work as an intervalometer automatically taking a series of 30-second exposures that later can be stacked (good for star photogaphy). A key difference to more expensive units is its range of 15 feet. Some "pro" units will work at several hundred feet but can set you back about $450 (like Canon's).
The hahnel remote that others have mentioned can be found at http://www.hahnel.ie …motecontrols&id=80&pId=80. This one works at a much longer range than the satechi and has some expanded capabilities. B&H has it for $99.
Vello FreeWave Plus Wireless ($60) at http://www.bhphotovideo.com …Plus_Wireless_Remote.html can control 16 cameras so long as you can find additional receivers.