[link|http://www.senate.gov/civics/constitution_item/constitution.htm|Article 2, Section 2 (paragraph 3?):]
"The President shall have Power to fill up all Vacancies that may happen during the Recess of the Senate, by granting Commissions which shall expire at the End of their next Session."
Now I know lots of people hate what the Constitution actually says, but it seems pretty clear to me. You can use recess appointments to fill vacancies that occurred during the recess. I haven't found anything that would indicate the contrary.
I know some, and I'm pretty sure all, of Clinton's recess appointments violated this. Bush's current appointment does. I suspect most of the recess appointments in the last 50 years violate this. But, if you have people who use the "equal protection of the laws" from the 14th Amendment to prohibit discrimination by race (segregated schools; good decision) and the exact same words to require discrimination by race (affirmative action; bad decision), then I guess all bets are off.
Brian Bronson