Ok, i did more testing (by the way, I should state i'm a voice expert). Sorry for the techinal info within the comment below, but it will further state my point.
First, when i tested the iPhone5 on my car, i was not using a 'cheap' car kit, rather it was connected to car's internal Bluetooth (Mercedes CLS 550). I also tried on my wife's car, Acura MDX, with same results. I then connected my phone to my Bluetooth test equipment. Basically this equipment connects any mobile device to Bluetooth and also supports PSTN connections. The test euipment supports call control as well as send/record voice for Voice Quality Testing (VQT). The test equiopment hardware includes a Cambridge Silicon Radio (CSR) Bluetooth Chip, similar to that found in most all Bluetooth headset devices. The Voice Quality algorithm used for this test was ITU-T P.862.1 (PESQ). The scale for the PESQ algorithm is 1-5 (5 being perfect). For all tests, the iPhone is on ATT network whereas the far-end is Verizon PSTN. Each test consisted of 3 different calls, each call sending/recordng 4 voice files. After each test i averaged all PESQ scores.
The average score for iPhone5 without Pebble was 2.71. This is average for mobile to PSTN.
The average score for iPhone5 with Pebble was 1.36. This is considered extremely low.
The average score for iPhone4 without Pebble was 2.40.
The average score for iPhone4 with Pebble was 1.22.
I saved the recorded voice files and, if you listen, you can hear a harsh static (which is why the PESQ scores were so low). This was a lab based test which confirms the Pebble is causing some degredation. I performed a few more iterations of these tests. For instance i unpaired the Pebble to the iPhone5. Repaired, reconnected, and then retested. The scores were much better initially, then got worse. It seems the Pebble is trying to pair to the phone via A2DP and not simply data. I also have an issue that the Pebble shows up on my iPhone as an Audio source (this doesn't make much sense).
Anyway, these are the results and the reason why the Pebble is causing me issues. This should in no way be considered an issue of the Bluetooth headset (or car kit) or an issue of the iPhone5. Pebble should work with the device, rather the device trying to work with Pebble. All comments are welcome.