One could just as easily say that Bush has kept America safe from the Chinese since Virginia Tech.
Ask yourself this... If the American borders are so porous that every Juan and Maria can not only walk into America for work, but back into Mexico for their holidays, what's to keep Muhammed and Abdullah from strolling in just as casually, exploding vests and all?
Basic logic would dictate that you can't have it both ways. Either America is not really overrun with illegal immigrants, or the terrorist threat is very nearly insignificant.
Bush hasn't kept America safe from anything, although he has done a mighty fine job of keeping Halliburton and Blackwater's bottom lines safely in the black.