ReadOnly field saved with NULL value
We have two values like client side and server side. In Server side coding done like float value have 0.0 etc. Read only field doesn't take value from the Client side because it's read only. In View, we see that 0.0 for float value because of server side coding. If you remove read only attribute, you can get value from the Client side and that value pass to the Server and store into the Database. Field with read only attribute, can't get value from the client side and store NULL into the Database. 
Hope this will help you.
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 ?