It seems to be more complex than this. Based on the tables from the datasheet, the lifespan gets significantly shorter if CPU runs more than 1/4 of time at 600MHz. I guess in reality there are no such big jumps in lifespan when going from 1/4 to 1/3 600MHz percentage for example and the tables give only rough approximations (with a good safety margin), but still they show some overall picture.

Another interesting thing is that the expected lifespan is longer when running constantly at maximum 600MHz clock frequency, than jumping between different clock frequencies arbitrarily. Maybe it's somewhat similar to lightbulbs lifespan (they also fail faster if you turn them on/off frequently, than just having them permanently lighted).

But the point is still the same. If most of omap chips can run safely at very high clock frequencies, I guess TI would bin them and sell really lots of high speed chips, blasting the competition (snapdragons, etc.). This does not seem to be happening.