Doesn't the contract specify they agree to use Washington as the jurisdiction for any dispute between them?
I don't know, does it? ;p. If you read that, then that's where the lawsuit will be. That actually is another sign that Mt.Gox may not have been as big into business as Coinlab, something I suspect is a big part of the problem here. Mt.Gox grew very organically, and I'm just not convinced Mark whoever is an experienced businessman. While you wouldn't think that matters, when it comes to contracts, it's easy to 'get one over' on the other party, which is why everyone has buttloads of lawyers to review these things.
And, yea, I'm sure Mt.Gox regrets the deal, BUT they probably wanted isolation from the possibility of US and Canadian regulation. They hedged their bets to decrease risk, and then - seemingly - reneg'd when the risk abated.