I don't think you need an ssl cert for testing but for sure before going live because you don't want users entering credit card data in an insecure form. But with AIM we are making a server to server post using ssl and that only requires an ssl cert on the authroize.net side.
Its easy to generate a self signed cert in IIS for local testing but of course browsers will give warnings about it becuase the cert is not signed by a trust authority.