I understand the mechanism of selection, but had never before heard of it re herbicides. After some digging I did indeed find a few references to studies of glyphosate tolerance.
A few things occur to me. I'd expect the process to take a relatively long time, compared to antibiotic resistance, due to the fact that in the time one generation of weeds develops perhaps hundreds or thousands of generations of bacteria will have passed. Second, although certain practices may exacerbate the problem, the process seems inevitable; anytime herbicides are applied there'll be some area at the fringes of the application where the dose is sub-optimal.
Ned Simmons