Nevermind, I resolved the problem.
It looked like the 1.21 firmware my unit was using was some kind of beta, since the date for it didn't match the date for the 1.21 firmware on D-Link's site. So I decided upgrade it....only to find that I COULDN'T. For some reason my unit wasn't accepting the new firmware (it gave no indication that the update failed either, other than the version number not changing after rebooting). Fortunately I found this thread:
http://forums.dlink.com/index.php?topic=11987.0
Which resolved the problem. I upgraded the unit to the 1.21 (2008/11/13) firmware, and now my Port Forward Settings are saving properly.