I have a 2011 328i coupe and use only the cable that came with my iPhone. No Y-cable required. Something must be wrong with your car or iPhone, because the standard iPhone cable it is supposed to be the only one needed in 2011. The statement below is from the BMW bulletin regarding 2011 model year production that began in September, 2010:
"iPod/USB Adapter (6FL)
This option has also been enhanced with many exciting features.
iPod One-wire integration: No longer will an iPod connection require the “Y-cable”. The original iPod USB cable is all that is required to connect the device to the vehicle. This simplifies the connection by not requiring the aux-input. The music is transferred directly from the iPod digitally to the HiFi system via USB. Therefore as of September, the Y-Cable will no longer be included with iPod/USB Adapter (6FL)."
Ask your dealer why yours isn't working.