[neutralino]

I think the point attempting to be made above is that Einstein called the cosmological constant his "greatest ever blunder" because initially he put it into the field equations to ensure that they had a static solution (the universe was thought to be static at that time) but then later took it out when Hubble showed that the universe is expanding. He believed that he should have trusted his theory, and not the general concensus at the time, which is why he called it his biggest blunder.

I think, although I have no proof of this, that Einstein was not talking about the cosmological constant as his biggest blunder directly, but the fact that he did not trust his own theory.

Nowadays, cosmologists are introducing the cosmological constant to provide the acceleration in the expansion of the universe. So, the points made by both Rascallpuff, and baudrunner (in his first post) are correct.