I just posted an article about Airspan and FREEDOM4 (a UK WiMAX operator) demoing a USB dongle that allows a user to roam between WiMAX networks that use different frequencies.

http://www.muniwireless.com/2008/12/01/airspan-an...

This allows an operator such as FREEDOM4 to cobble together frequencies in different bands (2.6 and 3.6 GHZ) and make them available for seamless roaming. My understanding of the UK WiMAX frequency auction is that there's been a delay in auctioning off the 2.6 GHZ band and each operator only gets a tiny slice (not enough capacity). But if an operator can assemble a set of bands for roaming — that should theoretically give them a better product for customers.

Of course, the best scenario would be where one can roam onto any WiMAX network around the world, the way you can with 3G.