Since we will likely be getting at least some orders from foreign countries, Authorize.net's fields are insufficient. Specifically, not all address formats fall neatly into address, city, state, zip, and for that reason we prefer to have address2 as an option for customers who don't want to fill out city, state, zip. The only problem is that not only is address2 not included in the standard fields, but we are prohibited from adding a custom field and passing address2 that way. The PDF documentation says that doing that will get your account banned, which seems really stupid to me considering address line 2 is hardly going to allow anyone to exploit anything.
I called Authorize.net, they seem to think that it's not actually against the rules to pass address 2. I also called Cybersource, they say that not having the address fields passed properly (in the rare circumstance where that might happen under the above described system) isn't that likely to affect credit card rates.