What is the RADiuS? It's an Open Hardware Arduino-compatible shield for the easyRadio Advanced RF Module from Low Power Radio Solutions.
I've been using easyRadio for a little over five years now... it's a fantastic RF / wireless solution ( I'll share the story of how I stumbled onto it later ). All you need is power, ground, and two serial-capable pins and you have reliable bi-directional communication between microcontrollers, desktops, laptops, or whatever. No frustrations over pairing, no twiddles with configurations, and it's approved for use in Europe, North America, and Asia.
It's a handy little module to have in your toolkit!
Which is why I decided to create an Arduino compatible shield and share the design as Open Hardware. I typically have a couple dozen bare boards on hand ( pictured below ), or... you can produce the boards yourself! All you have to do is send the following Gerber archive...
http://www.quilix.com/downloads/radius/RADiuS_v1.0_gerbers.zip
...to one of these project / panel services.
http://mayhewlabs.com/order-pcbs
http://oshpark.com/
In a couple weeks you'll get your boards ready to populate. In the meantime - make sure that you have all the components needed to populate the board. Let me know if you have any questions - I'm happy to help!

RADiuS MG ( Mayhew Green )

RADiuS LP ( Laen Purple )
FYI, I've tested the Easy Radio with Arduino, Arduino UNO, Arduino MEGA, several Arduino clones, Netduino, and FEZ Domino. If it runs between 3 - 5 volts and can 'talk' serial... done deal.