USB + Audio cable?

[nippywiffle]

Hi,
Is it possible to get an iphone 3GS dock connector, to USB AND 3.5mm audio cable?

I see that apple has the full composite cable for dock connector to component video + stereo audio, but that would cause a mess in my car, and I'd have to use a phono to 3.5mm converter too. I'd really like to have just 1 cable running from my iphone, back to the location where my 3.5mm audio jack is, and the 12v socket in my car.

Any ideas?

Also, are there any negatives to using the dock connector for the audio, instead of using the regular audio jack on the top of the iphone?

Thanks,
Dan