ReadOnly field saved with NULL value
change your probability field to function field, and write a function (ex _get_probability). And keep the current probability calculating function as it is. Now default probability calculate function display the value and second function (_get_probability) will save the value.
On CRM opportunity form view, i added readonly="1" for probability field. When i saved, whatever the value of my probability, it's stored with NULL value.
Is it a bug on OpenERP ?