it will depend on what you are trying to do. I would not even place the arduino in the same category as the iphone or beagle when it comes to processing power and speed.
The arduino is by far the easiest to program for.
Also I do not think that the iphone is cheaper especially when you add in the 2 year contract with data plan.
What are you planning to do with it?
UPDATE: Also just a heads up....double posting (let alone quadruple posting) will not help you get your answers >:(