If Brawl comes out in Japan before it does in the U.S. (is this confirmed by now, or just speculation?), then I feel *very strongly* that hidden characters should appear on the Dojo pre-release.
Why? Two reasons.
Okay, picture release day in Japan. Naturally, word of all the hidden characters will blast onto the internet like wildfire, whether you want it to or not. But of course, we'll all have to rely on second- or third-hand information from Japan about who the characters are. Which, of course, means that every idiot with Photoshop is going to be trying to falsely confirm characters, and the frauds will be taken a lot more seriously than rumors are today. It would be hell. I believe that Sakurai mentioned in an old interview that he knew this would happen, and would prefer that people learn about the secret characters from his site with a bunch of fanfare than just learning about them from a forum or internet blog (sadly, the thread I'm thinking of got unstickied a while ago).
And as for the second reason? The other day, I was poking around in my Melee records out of boredom. I happened to notice that I unlocked the last character on my second day of owning the game. That's less than 48 hours. So, hypothetically speaking, if I started playing Brawl completely unspoiled, I'd have a constant supply of shock for maybe between two or three days. In return for those couple of days, I would have been stressed out of my mind for three months with no meaningful information from the Dojo. And let me tell you, I will have plenty of excitement playing Brawl one way or the other. The surprise of seeing a new character means a lot more to me now than it will then. Plus, looking forward to playing a character is a lot more fun than hoping they get in and being worried that they might not. And on that note, if I go in unspoiled, what will happen when I've unlocked the last character and see on the spot that one of my most highly-anticipated hopefuls didn't make the cut? It would be pretty depressing, and it would cut out a lot of the joy of playing my new game. If it's going to come to that, I'd rather get it out of the way beforehand, because crushing disappointment would hurt the new-Brawl experience for me a lot more than knowing the secret characters ahead of time ever could.
And I know I only said two reasons, but here's a third just as a bonus. If the characters are shown, those who don't want the spoilers have the option of avoiding the internet for a while. If they aren't, those of us who DO want to see them have no alternative.