It's all a risk/reward premium, and 10% is worth it to a lot of people so it must be 10% risky...
Cheers for doing this though, excellent to see it in action.
Once GOX bites the dust, I honestly believe the arbitrage will be left to sub-pennies very quickly as new exchanges come online that are more like "real-world" exchanges.
PS. If I wasn't paranoid about hvaing MtGox freeze the account, or my US Bank Accounts frozen, or closed, I'd call up a personal friend and say: "Hey you want 10% every two weeks? Give me a $1,000,000 cash. I'll prove it to you and I'll throw in $100k myself!" but like I said, your bank account gets frozen there goes $1 million of my friends money, and $100k of my own...and if that's easy to get 10% than you gotta think about the realization that the operation is REALLY RISKY