In a statement, he announces that:

[I]t has become apparent during this process that this will not work for me as I have found that on issues such as the stimulus package and the Census there are irresolvable conflicts for me. Prior to accepting this post, we [i.e. Gregg and President Obama]  had discussed these and other potential differences, but unfortunately we did not adequately focus on these concerns. We are functioning from a different set of views on many critical items of policy.

Good for Gregg, but honestly, what took him so long? This conservative Senator just realized that he has too many policy differences with what promises to be one of the most liberal administrations in American history? Did he sleep through the last election?