[UPDATE] Midway has issued a press release confirming that this roster is accurate.
Well, it appears that all of Midway's carefully timed information releases for Mortal Kombat vs. DC Universe have come to an end. Achievements for a pre-release version of the game have appeared on xbox.com and they reveal a few more things about the game. Or, at least, they reveal things about a pre-release version of the game. That's the catch--since it's listed as "Mortal Kombat 8," many of the character names aren't properly capitalized or spaced, and Shao Kahn's name is spelled as both Khan and Kahn in different places, it's easy to assume that this isn't quite the final achievement list.
The roster leaks come via a set of achievements devoted to completing each character's expert combos. Here's a list of the 22 characters that will probably make up the game's main roster.
Mortal Kombat
DC Universe
The other achievements confirm a lot of things that we already know about the game, like the presence of two story modes--one for MK and one for DC. But there's also something in there for "Dark Kahn! - Unlock Both Characters (50 points)." If I may totally speculate, this probably just means unlock Darkseid and Shao Kahn (each one probably serves as a boss in one of the story modes) or maybe something more. But the name of the achievement makes me wonder if there will be a key point in the story where the two evildoers merge to form some kind of ultimate final boss.
It's a pretty good-looking list that pulls together a good group of classic Mortal Kombat guys with a decently wide swath of DC characters, too. That said, I'm always wary when a character like Shao Kahn is playable, as he's usually presented as a boss that isn't as interesting to use as the standard fighters. But he's got "expert combos" just like everyone else in this list of achievements, so it seems like he'll be in as a full-fledged fighter.
Here's a link to the page on xbox.com that shows the achievements, but assuming this will eventually get pulled or the achievements will be hidden, the guys at X360A also have the list up here.