Think of margin as like your broker has given you an equal credit to the money you have deposited and yes there is interest.
For instance, if you have 2500 on deposit, most brokers will extend you credit of another 2500 for a total of 5000 that you can use but that is not the purpose of having a margin account.
A margin account is like a credit card with a 2500 credit limit that you never actually use to purchase something......you are just taking advantage of the benefits of the account and one of those benefits is immediate "settlement" (in your brokers eyes) of your funds vs. having to wait 3 days for your money to settle in a "cash" account.
Hope that helps,
JMO
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