Juggler - I had the exact same happen. Filed BK, Wamu never sent any reaffirm papers so nothing was signed prior to my BK being discharged. Then a few months later they stopped reporting my mtg and called it a "closed account". Nothing negative (I was never late), but closed nonetheless.
Now, they flat out refuse to report it, because technically (and legally) I don't owe the debt. And neither do you in this case. They definitely will not allow you to "reaffirm later". A Reaffirmation agreement is only good and enforceable when submitted and accepted by the BK court BEFORE you are discharged.
I would LEAVE EX ALONE! The fact this is reporting is a mistake, but a good one for you. The other kind of good news is that if you ever get in trouble financially, you can walk away from the mtg and not owe a dime. They cannot report it as negative item on your credit report either.