Will my Shure MPA-3C work with an iPhone4?
Sorry, it will not work properly. The MPA-3C is not recommended for the iPhone4 as there is an undesirable interaction between the MPA-3C microphone and the ambient noise sensing microphone that is a new feature of the iPhone4. This ambient noise sensing microphone is located directly next to iPhone4 earphone jack - look for a very tiny circular opening.
For the iPhone4, Shure recommends the CBL-M+-K accessory paired with Shure earphone model SE535, SE425, or SE315. This combination is also recommended for the iPad.