In a nutshell, you add a high-frequency, low amplitude signal to the low-frequency, high amplitude AC signal. Powered devices don't care about the added noise, and filtering can extract the high frequency signal without the low frequency component.
Probably due to the dangerous nature of mains power, I haven't seen any blogs or tech notes on how to implement it. However, an Amazon search for "Power Line Communication" yields 325 books, of which most appear pertinent to your question, and by much more qualified persons than myself.
It is possible. However, if you're using an Arduino, you likely don't have the expertise to create a well-isolated and well-insulated shield. No offense intended, I would be hesitant to try it myself, and I've had some training on the topic at a university. Also, an Arduino is meant to be handled, and, as Joby pointed out, it will definitely need a well-designed enclosure.
All things considered, you'll have a much more flexible, easier to create, cheaper, and more robust system if you simply use an Ethernet shield and buy a powerline ethernet adapter like the one you linked to. Do some searching, they're available for less than $30.
Slightly off-topic, but I would consider Power over Ethernet to be much more within the reach of an Arduino shield. National has a decent appnote/FAQ on their LM507X series of powered device controllers. This is something I'd like to try once I get some free time....
Your question seems to be contradictory -- you seem to be saying you want to "create a circuit" without actually "designing a circuit".
I'm going to interpret that as saying you want to "build a complete system, including designing a high-level protocol and laying out a few circuit boards and soldering integrated circuits to the boards and plugging sub-assembly modules into those boards", but you'd rather not "design and fab a full-custom ASIC from scratch" or "design something from dozens of discrete transistors instead of a chip" or "design and do EM simulations and construct a full-custom antenna system and get FCC approval".
I've heard that, at least for low data rates, that UWB can be produced using simple circuits using off-the-shelf chips that were common long before anyone ever heard of UWB.
Alas, I don't know any specific chips that you could use for the data rate you want, much less if there exist off-the-shelf modules using those chips, but I hear that such chips exist. Let me give you some links that might lead to those chips.
My understanding is that there is currently only one UWB standard --
WiMedia’s Multiband OFDM, standardized as ECMA-368 and ECMA-369.
My understanding is that "Certified wireless USB" and a potential future version of "Bluetooth" and a potential future version of "Zigbee" are higher-level layers on top of WiMedia's UWB standard.
My understanding is that there are several chip manufacturers producing chips that comply with this standard. a b
I hear that several other chip manufacturers are producing non-ECMA-compliant chips, including Pulse~LINK, DecaWave, IMEC, WiLinx, Wisair.
Presumably those chip use some other proposed standard or proprietary UWB techniques.
If you can't find an off-the-shelf module, and you find yourself looking for individual chips, I suspect that many of the chips developed for HomePlug might be usable as part of a UWB system.
Best Answer
The problem is the other way around: most of the general public doesn't want to buy homeplug compatible modem IC's and so most of the IC's are restricted to manufacturers like broadcom\qualcomm who mostly deal with higher volume customers. (Like the BCM30321).
Look for homeplug IC's and IEEE 1901 which is the standard. Homeplug (the not green version) goes to 200Mbps. (also the 200Mbps is assuming a good pathway, which is not always the case with home/commercial wiring and YMMV as this rate is negotiated and depends on the noise on the lines and distance between devices)
There are also some non monolitic IC options here
To actually implement a product around one of these IC's would probably take 2-3 people just because of the time and effort it would be a lot to take on.