Michael Ossmann set out to make Bluetooth sniffing cheap and easy. Because of the way that bluetooth works (watch the videos if you want the technical details) there will never be a consumer product with any kind of monitoring or sniffing mode. The only hardware options when he began the project cost around a thousand dollars. So, he began to design an "open source bluetooth sniffing platform" that was relatively cheap and portable. Having never engaged in any sort of digital circuit tomfoolery prior to this project, he started with knowledge of Ohm's law, V = IR. And, ended up with these nifty little dongles.
Ubertooth Zero is the original design. Michael called it a "wireless development platform". He was able to design and implement a device that would take the 2.4GHz radio data in the air and turn that into a stream of bits over USB to be logged and analyzed. To this end, he coded a plugin for the popular Wi-Fi monitoring tool, Kismet. Michael presented the prototype Ubertooth Zero at ToorCon 12 in October, 2010. The interesting thing about this series of devices, the implementation is flexible. All it will take to reconfigure the device to perform any number of wireless tasks is to compile some new code onto the programmable logic chip that's the heart and soul of the device. With the application of some highly esoteric knowledge and probably way too much time, this can become the be-all of Bluetooth devices.
Of course he didn't stop there. Michael continued to refine the design and came up with the more powerful, smaller Ubertooth One. Take a look here for the video presentation he gave at SchmooCon 2011. The Ubertooth One costs under a hundred bucks to build, it has more power than the original, of course Michael handily raised 53,000 out of an original 16,000 Goal on Kickstarter. A contribution of $100 netted you a completely finished board, just plug and go. Lesser amounts got you printed circuit boards or kits to build your own Ubertooth One.
This is the Ubertooth project homepage. It contains everything you ever wanted to know about the design and construction of the Ubertooth series of bluetooth monitoring tools.