Paul writes:
If that scenario is plausible, then why aren't the farmers and
farmer trade associations expressing deep concern?  Their
livelihoods are on the line and yet I am unaware of evidence
they are seriously worried about their fields being overrun with 
superweeds that are resistant to a wide range of herbicides. 

I don't know that they aren't. You say they aren't; perhaps they are. Because you are unaware of evidence is hardly proof that there is no evidence. You won't see what you don't look for. A very brief search turned up this:

Invasion of the Superweeds

American farmers’ broad use of the weedkiller glyphosphate — particularly Roundup, which was originally made by Monsanto — has led to the rapid growth in recent years of herbicide-resistant weeds. To fight them, farmers are being forced to spray fields with more toxic herbicides, pull weeds by hand and return to more labor-intensive methods like regular plowing.

Comment by Stephen Powles:

Can anything be done about herbicide-resistant weeds in U.S. crops? The short answer is yes. This starts with realizing that glyphosate — Roundup and other trade names — is a precious resource for current and future harvests. Glyphosate is the world’s greatest herbicide. In my view glyphosate is a one-in-a-hundred-year discovery that is as important for global food production as penicillin is for global human health.

Yet glyphosate is failing in corn, soybean and cotton crops in the American Midwest and South because of massive overuse. This is also happening in Argentina and Brazil. For some U.S. grain and cotton producers it is already too late: over-reliance on glyphosate has led to the evolution of glyphosate-resistant weeds, and alternative chemical and non-chemical solutions will be required.

However, for many, glyphosate is still working, and these farmers have the opportunity to make changes now to give themselves the best chance that glyphosate will work for future harvests. This will call for diversifying crops and giving glyphosate a rest by using other herbicides and non-chemical weed control tools that make sense. Diversity offers the best chance of saving glyphosate.

Glyphosate should be conserved for future harvests in the U.S. and world crops because without glyphosate, global grain production becomes more difficult. And that will have a large effect on the global food supply.

Stephen Powles, is a professor of plant biology at the University of Western Australia. He is also a grain grower and glyphosate user. 

http://roomfordebate.blogs.nytimes.com/2010/05/06/invasion-of-the-superweeds/

* * *

Professor Stephen Powles, FTSE leads a large research team focused on Australian crop science and particularly the issue of herbicide resistant crops and weeds. Research extends from evolutionary biology, through molecular and biochemical studies to practical on-farm management of herbicide resistance. Prof. Powles is the Director of the WA Herbicide Resistance Initiative (WAHRI), receiving major funding by the Australian Grains Research & Development Corporation and the Australian Research Council. Prof. Powles is one of the most highly cited plant scientists in the world.

http://www.postgraduate.uwa.edu.au/research-leaders/professor_stephen_powles

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