Hi Jon - Yes, read only fields do not have their viewstate stored after a post back so the only way you can use a value from a read only field would be if you had that brought up (like a SQL Default value or a default value) when the form rendered. This is a security precaution with JavaScript and browsers... because if you could manipulate via JavaScript (like with an application such as FireBug) and modify the value of that read only field, then it really would not be read only.
What I usually do in this situation is: Create a hidden field as well as a read only field or text/html span tag and then use the read-only field as a placeholder that the user sees, and the hidden field as the value that is passed over to Authorize.net etc...
Thanks!
-Chad