From the Authorize.net Forums at http://community.developer.authorize...m-p/3791#M3467 for those who haven't seen it:

"06-30-2010 11:39 AM

I'm sorry everybody! There definitely are strange things afoot with the SIM responses. We're looking into it right now. Please check this post for updates to the issue and for what to tell your merchants.

Thanks

Michelle

Developer Community Manager"

And a follow up post from a non Auth.net employee posted after:

06-30-2010 11:56 AM

I just talked with support. I was told that the change had to do with this:

http://www.authorize.net/support/pafaqs/

And as far as the support agent was concerned there was no "fix" coming. This is going to be there permanantly.

Love how the FAQ for what we needed to do still reads:

"We will have more information soon on what changes you need to make if you use the Server Integration Method (SIM), Simple Checkout, Customer Information Manager (CIM), Virtual Point of Sale (VPOS) or the Virtual Terminal. Please check back regularly as more information will follow, including how the changes will affect the Advanced Fraud Detection Suite (AFDS)."

Anyway, as karenb points out you'll have to strip the extra trailing "00" from the x_amount field because the hash they generate is based on the amount without the trailing 00.

If somebody at A.Net knows if this situation is going to change please let us know.