Well, chalk this one up to the WTF files.
Sanded down the negative lead and chassis ground, reconnected......nothing. Still had the flashing theft light.
Corresponded with Even Guyett (thank the lord for him) who eventually suggested that I try another key. Well, I had and have been using Andrew's latest key. I had some issues with it in the past which were resolved with a new circuit board. I put in one of my original keys.....theft light gone. Ok. Tried the second original key. Theft light still gone. Ok, the aftermarket one is sure NOT to work as it has to be the culprit. Andrew's key now worked. Seriously, WTF? Only thing I could think of was one of the original keys reset something (the theft deterrent system which was stuck in 'on' position for some reason) and now allowed all the keys to work.
Leaves me very uneasy though. I don't know if it was Andrew's key that was the original culprit? Full disclosure, I admittedly cycled the lock/unlock button several times before I tried to start the car this morning to check my passenger side signal lamp which had gone on the fritz earlier this month. Perhaps that 'locked' the theft deterrent system to the 'on' position?
Well, unless someone can confirm this theory, I need to either get rid of the aftermarket key (which I don't want to since I like it otherwise and paid good money for it) or always bring an original with me (which is just absurd).
Cliffs Notes: If your engine doesn't start and the theft deterrent light is flashing, try one of your other keys.