I don't know why the officials didn't call a penalty on Iowa for the MSU game, but as far as having to attempt the 'try down' as the XP/2pt conversion are officially called, I think OK4P is right.
(Note: emphasis is mine)
The ball shall be put in play by the team that scored a six-point
touchdown. If a touchdown is scored during a down in which time in the
fourth period expires, the try shall not be attempted unless the point(s)
would affect the outcome of the game.
Rule 8, Section 3, Article 2a of the NCAA Football 2009-2010 Rules and Interpretations
http://www.ncaapublications.com/productdownloads/FR09.pdf
During the MSU game, the score was 9-13 before the score. We scored a touchdown, making the score 15-13. If, for whatever reason, we had screwed up the snap or kick or what-have-you, and MSU had run it back for a safety, the score would have been 15-15 at the end of regulation and we would have gone into OT. During the LSU game (aside from the fact that the rulebook may have been different), IIRC we ended up leading by more than 3 points. Had LSU somehow scored a safety on the try down, it wouldn't have been enough to win or force a tie.
As far as the celebration issue goes, interestingly, it seems to be the 2009-2010 rulebook where they mention perhaps making the rule that we are discussing; celebrating before crossing the plane of the goal-line would negate a score. To watch the video again, I don't think Stross (?) celebrates until McNutt is well across the goal-line and has taken a couple steps. Granted, McNutt could still have dropped the ball, and the official hadn't yet indicated a touchdown, so I suppose the play was still live. However, it happened so quick that I doubt an official would have called Stross for excessive celebration or unsportsmanlike conduct. Also, Stross wasn't taunting, etc.
As for a penalty for the celebration that delayed the try down? There probably should have been one, as there was for the PSU game. I've not gone through the entire rulebook, as there is a lot in there, but a cursory search yields the following interpretation:
SECTION 2. Unsportsmanlike Conduct Fouls
Unsportsmanlike Acts—ARTICLE 1
Approved Ruling 9-2-1
I. Team B scores a touchdown on a kickoff return, and substitutes from
the Team B area, with no intention of entering the game, run to the
end zone to congratulate the ball carrier. RULING: Unsportsmanlike
act. Penalty—15 yards, enforced on the try or the succeeding kickoff.
Officials should note the numbers of the offending players, for
possible disqualification later in the game upon committing a second
unsportsmanlike conduct foul (9-2-1-a-Penalty).
Note that it is for a kickoff return, but I think it applies to any sort of TD scored (really, if you search for either 'celebration' or 'congratulate', it gives you this one rule). While many of the players congratulating McNutt were on the field and not 'substitutes', you had most of the team out there (as well as the media, IIRC). And as we've stated, the game wasn't over because we still had to go for the try. Theoretically, we should have been penalized.
That said, there are a
lot of rules in the rule book, and I wouldn't be surprised if there were exceptions to rules with regard to others, etc. Perhaps somebody who knows more about the rulebook can chime in.
