I had ordered some inexpensive wireless units from sparkfun some time ago. I have managed to get them to work, but I think their utility for robotics is pretty much nill. The Xbee modules I discussed earlier are a far superior solution. 
I have worked with both the 315Mhz and the 434 Mhz modules. There is very little difference between these. There is an detailed walk through on the Sparkfun site BUT this walk through makes these modules seem far better then they are!
Here are the things that I found REQUIRED to make these modules work:
- Use an antenna! 23 cm for the 315Mhz and 18cm for the 434 Mhz. The antenna must be oriented vertically. Any small perturbations of the antenna cause bad data.
- While the modules are rated for 2400 or 4800 baud, I could only get them to work reliably at 300 baud (after lots of time staring at the scope looking at how crude the output from the receiver was.)
- The receiver needs to sync to the transmitter. I found that sending a series of 10101010 from the transmitter allowed the receiver to lock effectively.
- If you don’t send something every 50 ms or so, the receiver drops low, when it oscillates again for the first time there is almost always noise which will corrupt that byte, so you need to keep sending data.
- T protocol works better then N.
- Use a separate regulated 12V supply for the transmitter. By increasing the voltage to the transmitter, I was able to get the range to 10 feet!
Even with an afternoon of fighting with these, I could only get them to transmit about 10 feet line of sight (rated for 500!) at 300 baud. The reciever is also very sensitive to noise, so don’t plan on running any motors or servos in the vicinity.
Save your money and buy Xbee transceivers.