First let me say that I don't speak for all atheists. Second, I reiterate that none of this has anything remotely to do with science.

"Other than virtue--moral excellence--being its own reward, what motivates atheists to be virtuous?"

I guess I think that's enough for atheists.

"I presume rational atheists and theists, have a fear of breaking the law of the land and facing punishment in this life.. But only theists have a fear of facing some kind of judgement after death."

All true. Here's the thing: neither punishment in this life nor the next is entirely sufficient to keep people from doing evil. Of course, in my view 'evil' is a human perspective - and because of the law of unintended consequences we need to be very careful how we apply the term.

There is a sense, however, in which this stuff (minus the mystical junk) could eventually be amenable to some scientific analysis - and that could very well be an outgrowth of economic theory. Econ started out as a (very) soft science, but has really made dramatic progress in the last few decades. Nowadays there exists a body of theory that might permit analysis using techniques from my own chosen field of science, complex adaptive systems (CAS).

There is some interesting work that has been done to examine the effect that rules and policies have on organizations.
Check out the first four paragraphs of this Scientific American article:

Also, the first 3 paragraphs of the following Harvard Business Review article:

I met Dr. Bonabeau when he came to speak at a CAS group I started at work. He's brilliant, but he's not the only person doing first rate research in this subject. I've fixated on his articles only because 1) they don't have a lot of the mathematical baggage of a lot of the CAS work that gets done and 2) they clearly convey the sense that these techniques (agent-based modeling and genetic algorithms) are getting to the point where they're able to support experimentation into 'higher level' problems (i.e. policy).

The potential linkage to ethics is pure intuition at this point. I'm looking at the possibility that some of these ideas might be applicable to examining policy issues or organizations a bit larger than corporations: government agencies or entire governments. I'm attempting to develop a research program for examining these issues, but it's a bit of a hard sell.