WiMAX falling back to EDGE/HSDPA, maybe--then Nokia could sell it as an unlocked GSM device anywhere in the world. EV-DO and CDMA would be too limited (just the US, South Korea, some parts of China...). There's just no way (in the US, at any rate) to sell a CDMA device without getting the carriers involved.
As for a WiMAX modem for current tablets, wouldn't that just create a bottleneck when transferring data from the modem to the tablet? AFAIK WiMAX is significantly faster than either Bluetooth or USB, so it would make as much sense to just stick with using an EV-DO or HSDPA phone as a modem.