I sent an email to Kurt Shintaku to ask him about how the use of a flash drive for Readyboost would impact the life of the flash drive.  The response I received was unexpected.  Apparently the Microsoft folks thought about ways to reduce the negative impact of using a flash drive for such dynamic data...

Matt Ayers PM for Windows Client and ReadyBoost, responded to this question in many public forums:

Q: Won't this wear out the drive? (referring to ReadyBoost usage)
A: Nope. We're aware of the lifecycle issues with flash drives and are smart about how and when we do our writes to the device. Our research shows that we will get at least 10+ years out of flash devices that we support.

So fear not, using Readyboost is still a great idea to boost your computer performance and it won't cause an early death for your flash drives!