Users around the internet are noticing issues with their iOS device's ability to playback audio in their car while their device is connected to the stereo via USB playback. They say the problem came after they updated to 4.1.
Some users report a crackling noise, and others say that the music simply seems to mute itself within the first minute of playback. After that problem comes, they have to physically unplug the device and plug it back in to get it working correctly again, but it will mute again within a minute anyway.
As with any issue of the sort, it's hard to call it an issue at first, because we may just be seeing a few isolated incidents. Unfortunately, it seems pretty common right now and it all seems to come with the same update and people using the same feature.
We will see if Apple releases a minor update to fix the bug.