iLounge reports that there are noticeable differences in the audio quality of the microphones in the three main iPad 2 models: WiFi-only, AT&T 3G and Verizon 3G. They report that the WiFi-only model provides the best audio quality followed by the AT&T 3G and Verizon 3G models in that order.
I have a Verizon 3G iPad 2 and used the SoundNotes app to record Mini-podcast 22. I used its integrated microphone to record the first segment of the podcast. Then, I plugged in Apple’s In-ear headset with microphone to record the second segment. In my opinion the overall sound quality of the segment recorded with the integrated microphone sounded a bit better (fuller) than the sound recorded with the In-ear headset.
As I mention in the podcast, I do not have an iPad 2 WiFi-only or AT&T 3G model to use for comparisons. However, I did not find the sound quality of the recording to have any significant problems.