From the Associated Press:

What will this accomplish? Probably nothing.

Latino activists were already voting for him anyway, as the lesser of two evils.

Strong opponents of illegal immigration were already going to vote against him, so that's a push as well.

We doubt that it will even mobilize his left-wing base.

Perhaps some of those 800,000 will do some campaign work for Obama (making phone calls, placing signs, etc.), but that's speculation.

This move just does what all of Obama's other election year shifts have done: It makes him look desperate.