I in Delhi, India faced similar issue of 4S latching onto to a weak 3G signal resulting in crackling & dropped calls, frozen internet connections, poor battery and constant switching between 2G and 3G networks. My daughter's iphone 4 with its 3G switched off performed brilliantly on 2G network. This was happening in the prime location of Delhi on Vodafone. As Apple in India does not allow returns, I had no options but to get the carrier to de-activate the 3G network for my SIM. Now my 4S stays on 2G and works comfortably. Even SIRI works, accent issues not withstanding.
My experience showed that iPhone can switch automatically to a 2G network when the 3G signal become very, very low. It is quite possible that this threshold of change-over for GSM is determined by iOS and not by Qualcomm. If it is true then it should be possible for Apple to provide, for GSM networks, a software switch to increase this threshold to a 3G signal level of 100% - thus ensuring that phone doesn't go into 3G mode. To save battery, Apple could also modify the phone's preference/pinging for 3G network when this switch is turned on. Thus even if Apple cannot turn the UMTS baseband off due to this single GSM-CDMA chip design issues, the phone would stay in 2G network.
I agree with Ashley that nobody but Apple can confirm or deny this option though it does appear they would not do it untill this issue snowballs to another antenna-gate. It would be a major embarrasement for Apple as it would not only mean that they are admitting to a major design mistake but also that the phone may stay compromised due to sub-optimal battery performance (UMTS baseband staying on issues).
It could also mean that they are staring down a major recall - people not affected would also like to get their 3/4 months old phone replaced with a new one.
Finally, I just remember reading in a forum that a guy in UK who had bought his phone from US recently was told by Apple, calif. that he should return the phone and repurchase after 2/3 months, from a new lot of iPhones. Hope someone can verify this