So I have an N810, but what exactly is the WiMAX tablet and will it be possible to like "upgrade" the N810 or would you need to buy a whole new tablet for the WiMax?

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.

Bear in mind the area covered by wimax will probably be fairly limited to begin with, and no one has announced anything definite so far.

It's slightly more than a rumour in that Nokia did say last year they were doing a wimax tablet, but there were no firm dates or anything like that, and they only ever said that it would be a normal tablet with a wimax chip added to it..