According to their website, they pay interest on the commodity account. From what i understading, assume you go long 10 ES and the overnight margin is $25000, they take $25000 from your stock account and move it to the commodity account. So does the $15000/$25000 of margin you put up for the 10 ES eligible to earn interest?
According to IB the Initial overnight performance bond on 10 ES is $45,000 with a $36,000 Maintenance. I am fairly sure they will pay you interest on balances above $10,000 even when utilized as a performance bond towards a futures position. Most other FCM's DO NOT pay interest in a commodities account unless it is tied up in a T Bill.
IB will NOT pay you interest on the first $10,000 in your comodities account AND on the first $10,000 in your stock account, for the rest you should receive interest (unless you hold different currencies - you will not receive interest on the first ~ $10,000 USD in each currency).