add 5-6% for likely voter vs all adults, and another 5-6% for biased polling questions. and he's right around 50%.
If you add 5-6% for the imminent capture of Osama, 5-6% for the swearing in of the new Iraqi government, 5-6% for the general ineptitude of Democrats, 5-6% for the stock market going over 12000, and 5-6% for those who refuse to answer polls then he's right around 75%.
Again...
you can't just fudge numbers for likely voters vs. adults, because it's not an electoral poll. Non-voters are allowed to have an opinion on the President's job performance. This is NOT a poll about who will win the next election. It's a poll about Americans' opinion of the President's job performance.
As to the biased questions, I don't see much bias in "How do you rate the President's job performance?"