the parent was talking about the fact that the signalling used by the 2 are different.
GSM and CDMA are not compatible and that is why apple took a stand with following the standard used by most of the world which was GSM instead of using CDMA which still hasnt got the same coverage as GSM.
This outside of the obvious no data + voice at same time issue.
Also, WCDMA != CDMA.
The names are similar but they are evolved seperatetly and are two different incompatible standards. Its better to refer to the GSM WCDMA as UMTS instead.
The GSM group decided to take up UMTS as a standard for deploying the high speed networks whereas the CDMA group went ahead with the EVDO standard.
AT&T and T-Mobile are both GSM based telecom, however they are working on *different* frequency band for UMTS (which quite frankly is a pain in the neck) considering that they are not following any of the frequencies used by the rest of the world for UMTS, which makes UMTS roaming very very hard to achieve. None the less, the 2 companies are GSM based and therefore the GSM services would work exactly the same everywhere (well as long as you have a Quad Band *GSM* phone)