While it's true that you can't copyright a game, you can copyright parts of a game. For example, the game's artwork or the official Scrabble dictionary. I haven't actually played Scrabulous, but if they used the exact same board layout, that might be copyright infringement. Or, if they used the official word list, that might be copyright infringement.

You can also copyright the text describing the rules of the game. But, I doubt they'd be so lazy as to copy that...

And, of course, you can trademark the name of a game. As you said, Scrabulous is probably too similar to Scrabble.

I think it's possible to legally make a Scrabble knock off (not a lawyer, though!)... but, if Scrabulous had used a non-derivative board, word list, and a completely different name would they have gotten so popular?