The audio port is definitely the right way to interface cheaply (and across different mobile phones).

If anyone's interested, there _is_ now a way to interface RS-232 devices from an iPhone using an Apple-approved cable:

http://www.makershed.com/ProductDetails.asp?ProductCode=MSRP...

From what I've heard, negotiations with Apple took a lot of time and were quite "complex". Also, the cable unfortunately costs 80$ (because of the custom, "secret" DRM chip enabling the interface). Which goes to prove that the audio jack goes a long way!