I'm not going to bother reading through all 9 pages, but I just saw the same discussion on Swamp Gas and I've come to the following conclusion:
It was a backwards pass.

The rules state:

Caught or Recovered
ARTICLE 2. a. When a backward pass or fumble is caught or recovered by
any inbounds player, the ball continues in play (A.R. 7-2-2-I and II, A.R.
2-23-1-I).

Exceptions:
RULE 7-2 / SNAPPING AND PASSING THE BALL FR-101
1. Rule 8-3-2-d-5 (Team A fumble on the try).

2. On fourth down before a change of team possession, when a Team
A fumble is caught or recovered by a Team A player other than the
fumbler, the ball is dead. If the catch or recovery is beyond the spot
of the fumble, the ball is returned to the spot of the fumble. If the
catch or recovery is behind the spot of the fumble, the ball remains
at the spot of the catch or recovery.

Fumble
Touching or Blocking a Kick
SECTION 10. Fumbling, Muffing, Batting,
ARTICLE 1. A fumble is any act other than passing, kicking or successful handing that results in loss of player possession (A.R. 2-19-2-I, A.R. 4-1-3-I and A.R. 7-2-2-I)."

DEFINITION OF PASSING:

Passing
SECTION 19. Passes
ARTICLE 1. Passing the ball is throwing it. A pass continues to be a pass until it is caught or intercepted by a player or the ball becomes dead.

d. A snap becomes a backward pass when the snapper releases the ball (A.R. 2-23-1-I).

The exclusion of advancing fumbles on 4th down does NOT apply to backwards passes, the backwards pass never 'becomes a fumble' because the rule clearly states that it is a pass until it is dead.

The snap is a backwards pass. If you couldn't advance a backwards pass, you could never run a fake - or hell, any play on 4th down.

This post was edited on 10/10/10 at 9:04 pm