WiMAX has two variants- fixed/nomadic and mobile. They are two 'completely' incompatible technologies. They could have as well given them two different names. Don’t expect your devices to ‘smoothly’ upgrade to one or the other. Don’t expect your network service providers to offer both technologies ‘seamlessly’. No matter what they tell you, they are not compatible. At the most, they can make them work the way they have made GSM and CDMA work together- and you know how that works.
Fixed/nomadic WiMAX is interesting to emerging nations. Lack of copper and lack of long 'last mile' allows wireless deployments, and fixed/nomadic WiMAX is ideally suited to be the best alternative- to both broadband internet and voice telephony. Again, if you have DSL and it is working fine, then you don't need WiMAX no matter what they tell you.
Mobile WiMAX is interesting to developed nations. It comes as the best possible alternate to 3G cellular. I don't think it will come as a replacement of 3G. It will come more as a complementary technology. Mobile WiMAX will allow some operators to offload the bandwidth, allow dual technologies to efficiently manage their traffic and spectrum, depending on location, application and mobility (as three governing factors).