There's an interesting article on Baseball Analysts using pitch f/x data to analyze ball blocking. The author uses pitch type and location data recorded to come up with an expected passed ball/wild pitch rate and compares it to the catcher's actual rates. Long story short, Gregg Zaun and Kurk Suzuki tie at -32.1 PB/WP (fewer is better) over two and a half years, Bengie Molina is second to last with +30.2 PB/WP (for lack of a better term, Posada is last a +36.4). On the whole, these represent about one win above/below average over the last the two and a half seasons. Miguel Olivo, one of the names being kicked around to replace Molina, has a virtually identical +29.9.