My own conversation with myself...
Well now I'm screwed.
I unplugged my drive from the AEBS -- since it didn't recognize the existing Time Machine data -- and reconnected directly to my computer. I can activate Time Machine, and fly through all data normally.
But when Time Machine goes to do a new backup, it thinks it has 90GB of data to backup. This bad because I don't have that much space on my drive, so it fails. And I only have 75 GB of data on my MBP's hard-drive, so I don't know where Time Machine is finding 90 GB!
So now this odd thing has completely hosed my Time Machine setup.
My advice for now: don't connect current Time Machine drives to the AEBS. It's asking for trouble.