purchase_view.xml

<?xml version="1.0" encoding="utf-8"?>
<openerp>
<data>

        <record id="purchase_order_form" model="ir.ui.view">
            <field name="name">purchase.order.form</field>
            <field name="model">purchase.order</field>
            <field name="inherit_id" ref="purchase.purchase_order_form"></field>
            <field name="arch" type="xml">
                <xpath expr="/form/sheet/div/h1/field[@name='name']" position="attributes">
                    <attribute name="readonly">0</attribute>
                </xpath>
            </field>
        </record>

</data>
</openerp>