Does anyone know how the 7250i (or any other phone for that matter) decides to show the "Fill All Fields" selection in Wallet?

I am developing a Wap site using ASP.Net Mobile Forms and have got the username and password input fields set to Ecom_User_ID and Ecom_User_Password, but no sign of a Fill All Fields.
I have tried adding a non-hidden Ecom_SchemaVersion, but that doesn't seem to help.