You're correct about the 30% withholding tax..... this will be automatically deducted in most cases. I have an account with Datek, and have had deductions for tax withholding.
When you open your account, or whenever, send in a W8BEN form - this is available online from the US IRS service (not a bad site, considering). This will get you out of withholding tax problems since you have to state where your residence is for tax purposes.....