Researchers from the Max Planck Institute (Germany) and several North American universities have been experimenting with Arabidopsis thaliana and found that each nucleotide (or "letter") changed once in every 60 million individuals. This means that in sufficiently large populations all possible mutations are present (60 million is for instance the population of any large European country like France or Britain). It also means, assuming that the human genome changes at the same rate, that today, with some 7 billion people on Earth every possible SNP should exist multiple times as novel polymorphism.
This explains why herbicides, that typically target single genes, become obsolete so quickly. It probably explains also antibiotic resistence among microorganisms. They suggest that to be long-term effective these herbicides should target several genes at once, so the chance of adaptative mutations existing already in the attacked plants (and leading to a resistant new variant) is minimized.
Not that I'm in favor of herbicides or unnecessary use of antibiotics anyhow.
Source: Science Daily.
Research paper: S. Ossowski et al., The Rate and Molecular Spectrum of Spontaneous Mutations in Arabidopsis thaliana. Science, 2010. (Paywall).