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