What are the principles of implementing Ethernet over power lines ? Is it reasonable/possible to create an Arduino shield which will provide both power & network ? (properly isolate the "dangerous parts") Mains is 220V 50Hz where I live.
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....
The big question here is what is your objective? If it's to DIY your own powerline ethernet adapters, I'd say don't bother. Working with mains voltages and working out how to make power line carrier work at those speeds is probably not worth it. Additionally, the cost of the whole thing will end up being way more than it would cost you for those DX adapters.
If, on the other hand, you've got some other lower data rate application in mind and PLC is your only way to get the signal from point A to point B then you could give it a shot. There might even be some preexisting chipsets/modules you could work with to bootstrap the process.
For a good general overview of PLC, check out: http://en.wikipedia.org/wiki/Power_line_communication
Standard mains voltage disclaimers applies. Be careful, don't get shocked.