One of the "American Conservative" bloggers made the same point recently. I can't find the post to link , but he said that Obama didn't promise amnesty and the Hispanics voted form him anyway.

So Obama really doesn't need to offer them anything, but if he wants to hang on to the center-right wingers that settled for him, it would be in his best interest to not pass an amnesty.

For the record - I disagree with the conclusion, but if I am wrong I will be happy.