If the store does not have paid "downloadable content", the following should work. If the store does include "downloadable content", one may need to change some additional settings accordingly (after the following code change).
Edit the class constructor in "includes/modules/payment/authorizenet_aim.php" (change is in red):
Code:
// Reset order status to pending if capture pending:
//if (MODULE_PAYMENT_AUTHORIZENET_AIM_AUTHORIZATION_TYPE == 'Authorize') $this->order_status = 1;
NOTE: This post by DrByte mentions this line (and the thread includes a brief discussion around this line of code).