But, technically, she really can't become SoS. Not legally.
It's because of a rather obscure clause in the United States Constitution (Article I, Section 6) called the Emoluments Clause, which reads:
No Senator or Representative shall, during the Time for which he was elected, be appointed to any civil Office under the Authority of the United States, which shall have been created, or the Emoluments whereof shall have been encreased during such time ….
Translated? The Framers didn't want members of Congress creating new jobs or giving raises to existing jobs, and then taking those jobs for themselves. It was kind of an anti-corruption thing (an "emolument" is just an old-timey word for "salary" or "compensation").
The problem comes because of an Executive Order dated January 4, 2008, in which President Bush ordered the salaries of Cabinet Secretaries to be raised from $186,600 to $191,300. And Sen. Clinton's current term runs from 2007 through 2012. So you've met the preconditions of the Emoluments Clause:
1) a raise to existing "civil office" job (Secretary of State);
2) said raise was created during the time Clinton was a Senator; and
3) Senator Clinton now becoming Secretary of State.
Therefore, we have a constitutional problem.
Not surprisingly, this has come up before. In 1908, President Taft sought to have Senator Philander Knox (Note: "Philander" is a name that has gone curiously out of style, hasn't it?) appointed as his Secretary of State. But the pay for that office had been hiked the previous term.
How did they get around it? The Senate passed a bill which rescinded the previous pay hike for Secretary of State.
In 1973, President Nixon nominated Sen. William Saxbe (R-OH) to serve as his Attorney General after the Saturday Night Massacre, but the AG's salary had been increased in 1969 during Saxbe's term. But Nixon persuaded Congress to lower Saxbe's salary to the pre-1969 level. And that's how the inside-politics term the "Saxbe fix" was born (although really, it should be the "Knox fix", since Taft thought of it first).
And it happened again during the Clinton Administration, when Bill Clinton sought to confirm Senator Lloyd Bentsen as his Treasury Secretary. The solution? Congress employed the "Saxbe fix", thanks to a bill offered by Senator John Glenn.
And now we have it again today.
Admittedly, one can argue that the Emoluments Clause is "anachronistic".
Alternatively, one can argue that it doesn't apply, since the pay hike here came from an executive order of Bush, and not from Congress itself (therefore, nobody can claim there was "self-dealing" by a Senator or Representative, which is what the Framers sought to ban through the Emoluments Clause).
But if you read the language of the article closely, it doesn't matter who ordered the pay hike.
So, anachronistic it may be — or non-applicable to the original intent of the Framers — we have here, strictly speaking, in the plain words of the United States Constitution, a situation which is unconstitutional. Hillary simply cannot become Secretary of State, as things stand today.
The so-called "Saxbe fix" could be employed yet again. Yes, some might argue that the "Saxbe fix" doesn't render the unconstitutional appointment suddenly "constitutional". After all, they would argue, the three conditions above have been met, even if the pay hike is later rescinded. And such an argument might prevail. But no court has ever rendered judgment on this constitutional issue.
But, in any event, the absence of the Saxbe fix will definitely mean that Hillary's appointment is unconstitutional. So somebody in Congress better get a move-on, and offer a bill to rescind Bush's pay hike to the incoming Secretary of State.
And let's hope nobody makes a big fuss.