I just confirmed with CSR that you can fund the account with a small amount. The bonus is based on the balance from 6/15/10 to 9/15/10. Just make sure to transfer the money into the account by 6/15/10. You can also withdraw funds on 9/16 without affecting the bonus.
Please refer to my WTDirect $500 bonus review
for more details. I'll have more updates tomorrow.