Hi Bryan,
It should works with any kind of SSL cerificate because the certificate is required for the dialog between the customer browser and your website. On the other hand, if you read the AIM Guide you will found in the minimum requirement section:
• The merchant must have a U.S. based merchant bank account that allows Internet transactions.
• The merchant must have an e-commerce (Card Not Present) Authorize.Net Payment Gateway account.
• The merchant must have a valid Secure Sockets Layer (SSL) certificate and their Web site must be capable of initiating both client and server side SSL connections.
• The merchant’s Web site must have server-side scripting or CGI capabilities such as ASP Classic, C#, Cold Fusion, Java, Perl, PHP or VB.Net.
• The merchant must be able to store payment gateway account data securely (for example, API Login ID, or Transaction Key).
In other words, it's a legal requirement.
Gilles