The WiMAX tablet doesn't exist (officially at least). It is currently a rumour that several sites are debating. http://tabletblog.com/2008/02/black-...sis-wimax.html

As for upgradability to, don't count on it. WiMAX would require different hardware than what's in your N810. Wifi and WiMAX are totally different beasts, and would also require a WiMAX provider to sign on to the deal, much like a cell phone needs service from someone.