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I did some poking around the interwebs, and found some good info. If you were to compare emissions of a standard gasoline engine to a lean burn engine straight from the exhaust with no cat, the lean burn is much cleaner. Lean-burn actually creates less NOx and CO2.
The problem comes with the catalytic converter. Normal gas engines operating at 14:7 produce the right "mix" of gasses that react inside the catalytic converter. This reaction significantly reduces NOx emissions. With lean burn, the mixture of gases entering the cat is not ideal for oxidation because of the already lower NOx and CO2. Therefore special cats and other emissions control systems are required to further reduce emissions inside the cat.
So lean burn isn't the problem, it's that catalytic converters are more efficient at cleaning up the dirtier air from regular burn than they are from lean burn, so you end up with worse emissions after the cat on lean burn engines unless expensive new cats and emissions systems are implemented.
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