A lot of the buzz on the next Secretary of State has been John Kerry. This is even considered a safe choice by the Huffington Post. However, it appears that there is also some speculation that NY Senator Hillary Clinton may take the position. This is even referenced by folks like Alan Colmes. According to the Washington Post:
The pick of the former presidential contender and Senate Armed Services Committee member would go a long way toward healing any remaining divisions within the Democratic Party after the divisive primaries.
So will it happen? It looks unlikely, considering she's been edging for the Supreme Court... but Obama may just offer it so that she can reject it.