Redpark cable for arduino, any alternative
Apple are notoriously strict on what they allow to interface with their 30-pin port. I would be surprised if a makeshift system like what you have suggested works. But I am not confident that it wouldn't.

If you are interested in interfacing with an Arduino, would you absolutely want to go to a serial connection or would you want a direct connection to the ports? If you are looking for a direct connection I would think it would be more direct to get a breakout board like the PodBreakout from Sparkfun. 



It is much cheaper than the Redpark cable and then you could directly connect only the cables you need to.