I've been thinking about the same issue lately as well. I've found that the cheapest (and probably also the most efficient) thing to do is to just replace the stock head unit with an aftermarket one with an AUX input. I agree with GregFarz78 Those iPod adapters others have mentioned seem too pricey for what they offer. I may just end up going with an FM transmitter though as I'm pretty lazy and a clean installation might take over 45 minutes heh.

 

There are tons of iPod interfaces that you can choose from.

IMO you're better off spending the money you'd use on an interface and applying it to a replacement HU with a built-in iPod interface. Though that quickly becomes a slippery slope once you start adding in the other bits like a DCI, etc.