Over at Apple Support forums, a large number of users are complaining that their car stereos are no longer working after upgrading to Apples iOS 4. Users report that after updating an iPhone or iPod to iOS 4, plugging the phone into the cars integrated stereo create errors indicating the device is "Not compatible".
Here're some messages from the fourm: "I've a JVC DVD Touch screen (AVX820) - the player willn't recognise the iPhone with update software when I plug it in - tried another 3GS today and all works fine.
Used my 3Gs/3.1.3 quite well for months with my iPod car kit in my Volvo XC60. Upgraded to iOS4 and it doesn't work anymore. It says "USB not readeable", iPod app plays perfectly standalone. Help !"
More Info: Apple support fourm