Just thought I should add - in shopping cart management/admin area, if you see AIM Protocol (or similar reference to AIM), it's the same thing as authorize.net. Some carts have the configuration for authorize.net listed as authorize.net, others have it listed as AIM.
edit to add clarification - basically speaking, authorize.net has two different modes you can set it to on authorize.net side. AIM mode and regular. If I recall correctly, once you upgrade to AIM, you cannot switch back.