Maybe we could find out using a little bit of statistics. :

During the second world war the allied forces used statistics to make an assessment on the amount of bombs and material Hitler Germany produced.
They did this by decoding the serial numbers on broken parts that remained after a battle.

All the Internet tablets have a unique serial number that is the same as its Wifi MAC address. Maybe we could decode the number and link it to the "n-th" produced N800 ?
Does anybody know more about the Mac address and what the different parts mean? (I assume that some part might tell me what wifi chip is used or part of the number is linked to the manufacturer? But I don't know)

If we know what the number is of the produced N800 of a random group of owners, than we can apply statistics to estimate the total amount. (I will be happy to do the calculations, but I don't know how to get the production number out of the serial/mac code)

This is assuming the random draw of users in a representative group . One might argue that the average number of early adopters is higher among Internet Tablet Talk members than the normal public, so that might influence the estimate.