First of all if you haven't yet check out the official announcement from Riot http://dominion.leagueoflegends.com/
Im an avid League of Legends player and my initial reaction to this was "Awesome! Finally some solid content! And all the Riot hype from this week said it was going to be released this week!" Then it quickly turned into "Oh great more Soon(TM) bullshit". Now to defend riot, they did say its tentative date was after PAX Prime AKA Aug 28th. But if you know Riot, that could all very well be a load of crap and this could just turn out to be more Soon(TM) Vaporware. I mean just look at their promise history: Magma chamber was promised back in Sept. 2010.. Observer mode... graphics update.... Stealth rework... Replays.... All Soon(TM) crap that never came through.
Bashing aside, the game mode does look fun. It sort of sounds like a rip from one of BloodLines Champions game modes, but from the (very short and not detailed) video it does look fun and unique. I am excited to see it in full detail and try it out when (and if) its released.