I didn't want to format just yet. I ran a chkdsk /x /f on the card and installed the Kies bloatware...
After re-mounting the card and connecting my phone to the computer:
Mannnn, I didn't unplug it. I swear.
Interesting, it seems to work just fine when I unmount and remove my SD card. I'm going to copy everything off of it, format it, then see if it works again.