The answer to the first question is: quite possibly, but with something like tuberculosis instead of the plague. Less immediately scary, but more harmful: resistant forms of more common bacteria will create serious risks when undergoing any sort of surgical procedure. We could get back to the days when your biggest risk of surgery was dying from infection afterward.
The answer to you second question is: no, we haven't really been working on it. But the big problem is that we can't just invent new classes of antibiotics overnight. If one of these strains takes off, things will get a lot worse before they get better.
The bigger problem than either is that AFAIK, few places bother to sterilize their effluent. I'm not even sure if San Francisco bothers to do that, actually...
http://www.newscientist.com/article/mg22029470.700-fresh-air...
Frontline did an episode that covers resistance and the lack of new antibiotic drug development. http://www.pbs.org/wgbh/pages/frontline/hunting-the-nightmar...
I read somewhere a while back that no one has really been focusing on new forms of antibiotics because the old ones worked, and it's not profitable for pharmaceuticals to research it, as it's the type of drug a person takes for a limited time only (aka non-chronic) so the profit potential is much lower than say AIDS medication. Not sure how much truth there is to that however...