yeah I always thought that even if it was masked under the "delays the game" thing that it was largely due to players starting to wear things underneath their football shirts to advertise other products/brands or to make political type statements.
I think the rule's ridiculous personally, like has been said, sometimes a celebration is gonna be big, why shouldn't it be? and the rule itself is insanely inconsistent. You see one guy get carded for a celebration one week, then another guy receive nothing for something almost identical.
If they stopped wasting time trying to stamp things out of the game that weren't a problem, and started thinking about the things that are (like an instant yellow for anyone telling a ref to card another player for instance) and how to eradicate them from the game, which wouldn't take much thought, the game would be in better shape.
As for the Forlan thing, I can't imagine anyone ever changed any rules because of thinsg relating to him scoring goals. Why bother, it wasn't like he got much chance to celebrate them in England anyway