That's too bad..
The only difference from my set us.. is I only have guest check out. I'm not a fan of registering for an account.. they forget their log in info. try and make new but can't because that email is used already.. and move on.. But you may need to..
I couldn't run any good tests on my Dev site with Authorized.net sand box account because any thing you put in will go through.
I have gotten 6 orders since changing this afternoon.. no issues..
I have it set to not ask visitors to register either, I am just testing with a registered account.
Although, this may not be an issue for someone who doesn't want a registered account, but if you ever user shopper groups, or give discounts to certain users, there will be a problem. If users want to register to see their order history, this will be a problem as well.
I just placed one order in Firefox then went to Internet Explorer and placed another order, and the IE order replaced the Firefox order for the given user.