crm_task/crm_task_view.xml

<?xml version="1.0" ?>
<openerp>
    <data>

    <record id="crm_todo_opportunity_add" model="ir.ui.view">
        <field name="name">Opportunities - Tasks</field>
        <field name="model">crm.lead</field>
        <field name="inherit_id" ref="crm.crm_case_form_view_oppor" />
        <field name="arch" type="xml">
            <data>
            <page string="Lead" position="before">
                <page string="Tasks">
                    <field name="task_ids"/>
                </page>
            </page>
            </data>
        </field>
    </record>

    <record model="ir.ui.view" id="project_task_crm_form">
        <field name="name">project.task.form.crm</field>
        <field name="model">project.task</field>
        <field name="priority">20</field>
        <field name="inherit_id" ref="project.view_task_form2" />
        <field name="arch" type="xml">
            <field name="partner_id" position="after">
                <field name="lead_id" attrs="{'invisible': [('lead_id', '=', False)]}" style="width: 80%%"/>
            </field>
        </field>
    </record>

    </data>
</openerp>