I'm working on a project which will need a one way wireless switch. Basically, the arduino will have a receiver and it will "listen" for a specific signal, and when it receives that signal it will trigger an action. All I need is a simple triggering mechanism, not two-way communication.
One way to do this is to use some other radio based trigger/receiver. For example, you can get radio based camera shutter release for about $20 on ebay, and hack up both the transmitter and receiver cables. For most cameras, a shutter release just has 3 wires (ground, focus, and fire), that when connected, tell the camera to focus or fire. While I tend to use Olympus myself, I believe some Canon and Pentax cameras use standard 2.5mm phono cables, which might things a little easier.
If you had line of sight, you could use infra-red, but that's not an option.
In terms of a second Arduino, that's an option as well. You could get something like an adafruit DC Boarduino or USB Boarduino fairly cheap. I believe Xbee provides wireless options.