Still. The kinds of people who don't know all the hidden characters, I imagine, are
1. People who just don't play Nintendo (at least not very much). Spiteful or not, non-Nintendo gamers can't be expected to know Wolf's in the game.
2. Amish
3. My parents
4. Little kids with parents who haven't gotten them Wiis yet, and can't use the Internets
5. Non-gamers
I just can't imagine anyone who would use the Internet not hitting these so-called spoilers in the first place. 'Sright: I don't see how Mr. Game & Watch being in the game is a spoiler. If you went right ahead and said a group of Mr. Game & Watches pilots the Halberd while it spews Shadowbugs everywhere (and the Shadowbugs are those purple clouds Primids come from that is actually extracted from Mr. Game & Watch's body) and most of them get blasted out of it and turn into a boss which Snake kills and Mr. Game & Watch joins the party, spoiling part of the Subspace Emissary, then maybe.
But you'd really have to completely avoid any Brawl-related discussion, period, to avoid knowledge of hidden characters. Not just on the Internet--in real life, too. Have friends? Can't talk to 'em. They want you to play the game with them? NO WAY, they have hidden characters (that you can unlock in a day or two if you're fast).
Conclusion: Either you don't care about "spoilers," you already know, or both.