It turns out that my discussion of the Constitutional ramifications of nominating Clinton to be Secretary of State was, while prescient, somewhat misguided in that it was directed at the wrong Clinton. Senator Hilary Clinton, it seems, faces an Emolument Clause problem. Article I, section six of the United States Constitution:
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 increased during such time....
In plain language, this means that no Member of Congress may be appointed to an office that was created, or the salary paid to which was increased, during his or her term in office. What does this have to do with Sen. Clinton? Well, back in January of this year, President Bush signed an executive order raising the salary of Cabinet officers from $186,600 to $191,300. And Sen. Clinton's term ends in 2012. Hence, she is Constitutionally ineligible to serve in Pres. Obama's Cabinet.
This is hardly the first time this problem has arisen. The Emolument Clause stood in the way when Sen. Philander C. Knox was nominated by President Taft to be Secretary of State; Congress "solved" the problem by passing a law reducing the salary of the Secretary of State to its level at the beginning of Knox's term in office. Subsequent Congresses did the same for other nominees in similar situations, including Sen. William Saxbe, nominated by Pres. Nixon to be Attorney General, and Sen. Lloyd Bentsen, nominated by Pres. Clinton to be Secretary of the Treasury. This kind of legislative action -- known nowadays as a "Saxbe fix" -- is arguably unconstitutional, but it's never been challenged in court. But if there's any Senator whose nomination might prompt such a challenge, it would be Hilary Clinton.