I think the proposition of a single WiMAX data network is just about dead. Hopefully the tech survives to drive things on the scale of municipal wifi projects - as it's a vastly superior technology than wifi in those situations.
But in any case, AT&T is firmly in the LTE camp.
that doesn't mean that Intel (who is firmly cemented in the WiMax camp) won't use its influence with Apple to get an WiMax-powered iPhone...
I don't doubt a WiMax-capable iPhone might happen, as a secondary data capability ala the existing WiFi support.
I just don't think it would be sold as a WiMax iPhone as opposed to GSM/CDMA/LTE.