I've had issues on Telstra. Sometimes the phone UI will prevent you from answering the call. Sometimes the phone crashes mid-call. Sometimes you answer the phone and you can hear the other party but they can't here you.
A similar thing happens on my Sony Ericsson S500 phone on Vodafone.
I don't know if it's the phone or the network - I haven't done much testing. It's quite irritating pressing the answer button repeatedly and the phone not answering the call, or when I call someone and I end up unable to hear them while they can hear me.
It may just be AT&T in some regions too. I have never experienced the level of frustration that seems to emanate from iphone users in the Bay Area (I'm in Oregon).
There were initial issues with Optus in Australia - their network wasn't prepped for the volume the iPhone delivered, so disconnections and poor connections were common complaints for the first 2-3 months. I believe that's been sorted now.
Apple chose them as their exclusive US carrier, so I don't see much sense in passing the buck. But we use different frequencies for the most part so it could still be a phone problem and you would not experience the same issue.
I've heard of a couple of complaints about low signal strength. (compared to other phones on the same network). (this was in Oz - think it may have been optus)
Possibly, but that's because Optus has a pretty sucky network for everyone. I understand (from reading whirlpool) that their new 3G network is not iPhone compatible as it's 900MHz
I think your information's out of date. I use Optus 3G on an iPhone in an area which was in exactly that position, but they upgraded a week before the iPhone came out. I think that upgrade happened nationwide. (Not that it's relevant to this thread, I know, but...)