I've been toying with the idea of doing the hack after I couldn't get a cassette adapter to work - however, I would've thought the iphone 4 would work with the normal ipod/dock connector? I had this system in my previous car and it worked, the only problem I noticed when changing from a 3GS to the 4 was a loss of charging capability.
Would be good if someone could shed some light on this before I go ahead with the hack...