Today in 2013, US Patent EP 2172402 B1 was issued, an invention of William P. Apps, assigned to the Rehrig Pacific Company, for his “Plastic Beer Keg.” There’s no Abstract, but they state in the description that the “present invention relates generally to large multiple serving beverage containers, in particular, beer kegs.” They also list 13 claims about why this invention is unique, but the main one, of course, is that these are plastic, which is far cheaper than the stainless steel ones that are used today, and are also prone to theft. It will be interesting to see if they catch on.
Likely not – high-quality food-grade plastic isn’t fire/extreme heat proof, nor is it 100% impermeable or tough to puncture (though it’s practical weight-wise).