I just intalled one in my desktop. I read a lot before purchasing. I would say the answer is... it depends. As lammert points out, the wear is on writing, not reading. In fact, as these devices "wear out" they turn into read only devices.
MLC devices will wear out after around 10,000 write cycles, while SLC devices last about 10 to 30 times longer.
With wear leveling, you can almost calculate how long the drive will last. That's why I say, it depends. But I'm guessing, they last a lot longer than people think...
For example, a 120 gig mlc device can last up to 1,000 terabytes of written data. Over the course of four years, you'd have to rewrite the entire drive 7 times a day, 365 days a year for 4 years.