JP you are correct, they are hub eccentric, but the truck is so light that is doesnt matter much.
Clavin I wouldnt be grinding the wheels but the hubs, and its so close to the center that it wont effect the balance.
Mike Im glad that grinding it worked out for you, I havnt decided yet between grinding the hubs or using the hole-saw method described here:
http://www.nissanminis.com/modshop.php?mod_cat=82
It looks like a good alternative to getting them bored but I have the steel wheels so it will take a lot more effect to drill them out then it would the alloy wheels.