As of today according to Sports Illustrated............

"The Padres have MLB's best bullpen ERA (2.75) and third-best starters' ERA (3.49) and it is on the strength of this pitching that San Diego takes over as the new No. 1. Not bad for a club that debuted at No. 24 in the first PRs of the year back on April 15, which also happens to be the only previous time a team other than the Yankees or Rays held the top spot."

Read more: http://sportsillustrated.cnn.com/2010/writers/joe_lemire/08/26/power.rankings.1/index.html#ixzz0xk3YvQnp

Of course, I knew this several weeks ago. :)