UPDATE: NY Times says it's Joe Biden for sure:
It reflected a critical strategic choice by Mr. Obama: To go with a running-mate who could reassure voters about gaps in his resume, rather than to pick someone who could deliver a state or reinforce Mr. Obama’s message of change.Obama gains some gravitas. Let's hope it frees him up to return to some of the more liberal, reforming themes that were giving him higher poll numbers at the end of the Democratic Primaries.
Mr. Biden is the chairman of the Senate Foreign Relations Committee, and is familiar with foreign leaders and diplomats around the world. Although he initially voted to authorize the war in Iraq — Mr. Obama opposed it from the start — Mr. Biden became a persistent critic of President Bush’s policies in Iraq.