Two or more players celebrating
in the endzone
after a touchdown
can be fined, but there will not be a penalty
assessed against the offense
unless the endzone-dancing continues long enough to constitute a delay of game
The rules on delay of game vary widely, depending primarily on when, in the game, the delay occurs. Delays in the last two minutes of a half are particularly frowned upon.
Apparently, excessive celebration by an individual player can be considered "unsportsmanlike conduct", but I'm not sure what the penalty (personal foul, team penalty, what-have-you) might be for that.
Orange Julius says: endzone dancing flag, "Personal foul. Unsportsmanlike conduct. 15 yard penalty enforced on the PAT."
Thanks to http://www.nfl.com and http://www.pigskinparlor.com/rules