Diagnosis: It seemed that Framemaker 9.0 randomly, for no reason discernable to me, began inserting question marks in place of apostrophes and double quotes. From the Adobe Character Sets PDF: "If a character glyph is not available for the selected font, Framemaker displays a question mark (?) in its place." Since turning off Smart Quotes resulted in me being able to insert regular quotes without incident, it appears as if Framemaker suddenly stopped recognizing automatic smart quotes.
The Fix: I'm embarassed to say this, but the solution was simply to install a clean maker.ini file. At some point (I have no idea how), it seems my maker.ini file became corrupt. I have no easy means of comparing the corrupt file to the uncorrupted file, so I can't say exactly where the problem exists, but I do know that I've used this fix to resolve the problem on several different machines in my office.
To all Framemaker users, I highly recommend keeping a known good backup of your maker.ini file for just such occassions.