telltommy, that's a good question and there seem to be lots of different answers out there on the 'net. It's not the writes that kill flash, it's the erase cycles. Of course, in order to write new data or modify the old, you have to erase first so it might as well be the same.
Flash memory "endurance", as it's called, varies widely. Inexpensive flash drives can have an endurance as low as 10,000 erase cycles up to around 100,000. See this article from Imation and a page from Kingston about their flash drives endurance ratings:
As Solid State Drives (SSD) start coming on the market, these endurance numbers need to be pushed higher -- up to 1 or 2 million and higher. Wear leveling is a technique to spread the erase cycles around the memory cells so they all wear out at roughly the same rate. This technology is still being advanced. Try googling on keywords such as "flash endurance", "wear leveling", and so on to read more.