The GameSpot.com to GiantBomb.com migration was/is a similar situation. Many of GameSpot's top editors left around the 'Gerstmann Incident' to form Giant Bomb. It hasn't had a long term effect on GameSpot's traffic, and despite great content, GiantBomb hasn't really taken off[1].

My feeling is that for established media style sites, unless the people breaking away are doing something _very_ unique, it's the URL that's more important.

[1] I'm not privy to their data and goals, but traffic wise they aren't getting closer to GameSpot. It's just my impression.