Magento's core code does not pass address line 2 to Authorize.net. This is a problem when Authorize.net is set to send email receipts, which get sent to the customer missing their shipping address apartment number, etc. (The Magento order confirmation email is fine.)
To replicate the problem:
1) Authorize.net is set to email a receipt.
Meaning, you log in to your authorize.net account > Settings > Email Receipt > Check the box for "Email transaction receipt to customer (if email address is provided)"
2) Customer places an order with address line 1 and 2, then pays via Authorize.net. FirstName LastName Address Line 1 Address Line 2 (apt 203) City, State ZIP
3) OH NO, the customer receives an email receipt from Authorize.net showing only address line 1; it does not show address line 2. This causes some customers to panic thinking their package will not be delivered correctly.
Update: I'll post an answer tomorrow. Not permitted to answer my own question for 8 hours...