Android as it should be known, will probably supercede the iPhone, the Iphone OS can only run on apple devices, Android can run on any phone hardware, provided the phone manufacturer developes the required hardware for the phone.
At the moment HTC are the only ones who have released a phone, it will only be a matter of time for the others to bring out there model.
However, Nokia seem to be holdink back, given they have their symbian OS, and I would imagine they are waiting in the wings to see how well android does before commiting themselves.
It should also be noted that, while the iphone 3g sold 3 million units in its first month and Android has only done 1 million todate, there has not been much advertising for the Android phone yet. I think that once they come out of the Beta testing phase and release the 'G2' we will see more networks adopting and promopting the phone. In fact we probably should not even compare the 'G1' to the Iphone 3G seeing as the how the '3G' was the second incarnation of the iphone and was rideing on that success of the first.
Time will tell.