One note about fees - because of the way this works with Bitcoin's multi-signature transactions, arbitrators don't have to intervene at all when there's no dispute so a lot of them choose to charge fees only for disputed transactions, meaning that most transactions should be free of charge.

That being said, I do see the rational behind charging for every transaction and think that its going to be a popular choice too. I wrote some of the reasons for that here: http://www.reddit.com/r/BitcoinSerious/comments/1sqibz/bitra... (near the end, in the "Edit:")

(full disclosure: I'm Bitrated's creator)

I absolutely think the only feasible business model for arbitrators is to charge a fee for each transaction. Otherwise, how can they afford to spend the time and money to investigate a dispute properly?

People are not going to want to pay for more than the cost of an item when there is a dispute, yet it could cost many more times the value of the disputed item to conduct a proper investigation and produce a fair arbitration.

Great project, by the way. Thanks for releasing it open source!

Does this automatically work with all bitcoin accepters? Or do they have to option / decide if they want to accept it?

They have the option to accept it. Technically, if you figure out the merchant's public key, you could "force" them into being a part of the "escrow" [1], but unless they agreed to it (and agreed to the arbitrator being used) they won't consider the product paid until the payment is under their control.

[1] I'm using quotes because the way it works with multi-signature isn't an escrow, but escrow is commonly used when talking about something like this. There's some explanation about how multi-signature transactions works and why its not an escrow in Bitrated's FAQ, under "How does Bitcoin arbitration work?" and "Is this an escrow?" (https://www.bitrated.com/faq.html)