I wired up the pair of NRF24L01 modules that I bought from Jean-Paul last Thursday to a couple Arduinos tonight. I found a library / sample code here and modified it to make an audible tone on a piezo speaker I wired up to give a nice ping every time a packet was received on the "server" side. I did a quick range test with the "client" set up in our office and walked outside down the street to about 185ft before losing the ping. I think with some proper antenna positioning, this could easily be extended.
I'm thinking about getting some cheap Arduino clones and some more transceiver modules to add some data collection around the house - little things like freezer / refrigerator temperature, garage door state, room temperature, furnace air-flow, etc. With some decent logic to tie it all together, this could make a good start to some home automation.
My next thought is making a small coin-cell powered telemetry unit for small quadcopters. Something like this for altitude (0.5m resolution) with a small micro controller and maybe some other sensors reporting back to a small simple LCD on the controller could be useful I would think. I'd also like to have the quad side unit able to receive a signal from the controller side to maybe trigger a solenoid (think parachute drop) or turn on some LEDs or something fun.
Hardware eventually fails. Software eventually works.