As part of their comprehensive review of the new "MacTablet," iLounge discovered that the iPad 2's microphone placement had changed considerably from the iPad 1G: from a round hole beside the headphone port to a pill-shaped cutout directly above the front-facing camera. Additionally, difference in construction between the Wi-Fi and 3G models means that the mic is in the metal housing of the 3G model (both GSM and CDMA), while in the Wi-Fi version, it's in the plastic antenna strip.
Further testing indicated that the Wi-Fi mic's audio quality to be cleaner than either of the 3G's, which sounded muffled and echoey by comparison. Interestingly enough, the CDMA's mic quality also proved to be worst than its GSM brethren. (Cue the Verizon nerd sputtering "Whattaya mean, you can't hear me now?!?") The problems are also at their most noticable when the iPad's screen is facing the user (e.g. a FaceTime session).
It's too early to tell if Apple will be faced with an "AntennaGate"-level backlash, or if they'll simply say "Hey, we're not George Martin, and that iPad's not Abbey Road studios — if you want higher quality audio, plug in an external microphone!"