I have a Fiat 500 with a USB connection. I use a Blue & Me adaptor to play my iphone/itunes through the speakers. After upgrading to OS4 the itunes play for a couple of seconds then the stereo reverts to the radio. Is this a similar problem to yours.
I have enquired with other users of Blue & Me adaptors some are having the same problem but others are not.
I'm experiencing the same problem on my Pioneer DEH-3200UB. Worked fine prior to the update (all controls on my steering wheel as well) now when I plug my upgraded iOS4 3Gs iPhone into the deck, I get an Error-11. I did speak with Pioneer regarding this issue and they said it was a problem that had to be fixed by Apple. I spoke with Apple support right after speaking with Pioneer and Apple support stated that they can't support every device out there, but they may have a fix for it, or may not, in the future.
Apple support continued to say that I should post feedback on the iPhone support section of the website as the engineers do read those. I urge everyone that has this issue to post feedback on the compatibility section of the iPhone support site to draw attention to this so it gets addressed quickly.
I asked the Apple support rep if there was a way to revert back and was told "No" that Apple does not support going backwards and has removed all the old OS software from their site so it can't be done ... <pause> .. by Apple. He did say that there was "sites on the Internet" that have instructions and ways to go back to the old software and continued to state that it would not violate the terms of my agreement. We'll see about that when I bring my brick into the Apple store and ask for a new phone.
Apple is responsible to fix this problem or provide a work-around, even if it is a way to revert back to the old software. There is nothing in iOS 4 that I can't live without, and it really comes down to the point that there is nothing on a iPhone that I can't live without.
I'm not going to replace my car audio deck every time Apple puts out an update - I'll replace my phone.
Call support and post feedback. It's the only way to get it fixed. In the meantime I'm researching Android/Pioneer compatibility.
Apple changed something about their USB protocol with iOS 4, and a lot of car stereos don't like it.
Crutchfield has been testing recent models with iOS 4 and noting which ones do and don't work in their "Hands On Research" writeups. It doesn't look good at all for Kenwood, Alpine, JVC, etc..
Some Pioneer decks still work. If it is a DEH-PxxxxUB model, chances are it will. Other models not in the "P" series, even new ones like the DEH-7200HD are reported to not work.
I have a Pioneer DEH-P5100UB and browsing my library from the stereo is now glacially slow: it loads maybe one or two items per second. When my 3GS was running iOS 3.1.3 it loaded up instantly. As soon as I upgraded to iOS 4, things got slow. Not such a big deal, I tend to select what to play from the iPhone anyway.
Other than that, audio is fine, playback controls (pause, skip, prev..) still work for iPod, Pandora, etc... and song info displays for iPod.
I also have a Pioneer head unit and experienced the exact same thing after iOS4. The worst part of it for me is that it actually works SOMETIMES! So I can be enjoy my music on my iPhone4 while commuting in my car for a couple of days and then without warning I get the Error-11 with no warning or reason. Another strange thing is that during the times I get the Error-11 I can SOMETIMES pass music through by starting a song before I plug into the head unit. I won't have any iphone control but i can hear music. Irritating!!