Update: We're getting reports the app is crashing for jailbroken iPhones -- and possibly non-jailbroken ones as well. We just got our first denial when dialing out over 3G via a popup notification, but it only happened once and we were back to old habits one call later. We did experience a brief period where no sound was coming through, but that might have something to do with using OS 3.0 and was remedied by restarting the phone.
Update 2: So far, it looks like dialing over 3G's only working for those with OS 3.0 beta, but unfortunately all of us in that situation are now suffering from frequent crashes and loss of audio. Things seem to be going smoothly for those running the latest non-beta firmware, but the calls only work over WiFi. That said, when using the app as intended (non-beta firmware over WiFi), audio quality was surprisingly good, far better than any third party apps we've used.
[Thanks to everyone who sent this in!]