I found an interesting article in Scientific American (Coates, 2005), and I quote part of it:
The condition of having no more than five fingers or toes [...] probably evolved before the evolutionary divergence of amphibians (frogs, toads, salamanders and caecilians) and amniotes (birds, mammals, and reptiles in the loosest sense of the term). This event dates to approximately 340 million years ago in the Lower Carboniferous Period. Prior to this split, there is evidence of tetrapods from about 360 million years ago having limbs bearing arrays of six, seven and eight digits. Reduction from these polydactylous patterns to the more familiar arrangements of five or fewer digits accompanied the evolution of sophisticated wrist and ankle joints--both in terms of the number of bones present and the complex articulations among the constituent parts.
So Coates states that around 360 million years ago there were tetrapods with more digits (6, 7, 8), but that this was reduced to 5 about 340 million years ago. Losing things in evolution is generally relatively simple, yet re-gaining them is hard. So while there is still a tendency for evolution to create species with less digits, there are no examples of land dwelling species with more than 5 digits.
The author reasons that more digits do no good on land as more digits impair locomotion. Most polydactylous species were indeed found to be aquatic.