I've setup a new instance of our development server on localhost, checked out the latest working copy, and was given a copy of a database backup to use with it. After getting all of this setup and working, I went through a demo transaction to make sure my checkout was working after reverting to the working copy.
It looks as though the checkout is actually trying to process my transaction on the development server now though, as it's declining the card with:
Error Code: 10544: Please use another payment card
I've set the System->Configuration->Payment Methods->Authorize.Net->Test to yes, but it's still trying to process my card it seems.