After writing this post on Google+, I felt the need to publish it here, as a sort of archive so that I could come back to some of the ideas within as a mine for fiction, non-fiction, or simply a good line at a cocktail party:
Rate of evolutionary change is a complicated concept because, at some level, you're trying to collapse a variegated gene pool into a single entity that can then be measured by the amount we perceive it to be different from its predecessors. Is that measurement to be phenotypic? Genetic? A combination of both? If you just measure genetics you end up ignoring the fact that a whole lot of change can occur to "junk DNA" or mitochondrial/commensals DNA without noticeably changing the organism. If you just do phenotype then you're going to miss changes that might result in speciation without phenotypical changes. Doing both requires you to come up with some kind of weight distribution that basically, as far as I can tell, is determined by your arbitrary bias.
A species can go a very long time without experiencing noticeable change or significant speciation if it experiences no bottlenecks or separations, but this is not to say that its gene pool is not diversifying or converging greatly. Similarly, a species at a bottleneck event may very quickly speciate, but its difference from the progenitor who survived the bottleneck may be little.
At the root of this problem is the issue that we don't have a clear definition for what "species" means. Reproductive compatibility isn't a binary equation, genes intermingle in an astonishing number of ways, and in general the problem is just very difficult: http://en.wikipedia.org/wiki/Species_problem
Speciation is, after all, a human attempt to define a natural system that could not possibly care less. From one extreme point of view, everything on earth is a single species, while from another extreme each genetically unique sibling is a separate species from its fellows. The need to keep researchers employed and the improvement of measuring technologies ensures that science will continue to tend toward the latter extreme, allowing new species to be discovered all the time. After all, it's ultimately just a question of how minutely you wish to split the hairs on evolution's hoary back.Needless to say, the same may apply to all cultural artifacts (language, technology, etc.), and of course even to the strata of the universe (differentiation of matter, energy, vacuum, etc.). This kind of cuts to the heart of science while preserving the usefulness of the body, in an odd sort of way.