What you mean by 'allowed'? I don't think there is any law against using a semicolon or any other special character in a business name. As you point out, many dot.coms do. And there's Yahoo! with an exclamation point. And the Toy'R'Us example already cited.
I'd say that if the semicolon helps accomplish your intent, I don't see why not. Of course, you can't use the special character in your domain name, so whatever you are doing should probably be able to stand as something recognizable in cases where you are not able to include the semicolon.
As others have mentioned, semicolons are not feasible in .com domains. That said, I think it all comes down to how translatable the name is to typing. If the semicolon replaces an i and it's pronounceable, then you shouldn't have any issues. Yahoo! naturally translates to yahoo.com, Email: Your Way would naturally translate to emailyourway.com. But something clever like M;ne would be a bit of a challenge; is it "Mine" or "Mne"?