System Config Multiselect default all selected

Use DataScript to store data in config module.

How to create data script

http://inchoo.net/magento/magento-install-install-upgrade-data-and-data-upgrade-scripts/

In datascript simply call the core config module and save your data

$myDynamicValue = '1,2,3';
Mage::getConfig()->saveConfig('section/group/field', $myDynamicValue, 'default', 0);

I give multiselect example.

<fields>
    <view_style translate="label">
        <label>Display Settings</label>
        <frontend_type>multiselect</frontend_type>
        <source_model>yourmodule/system_config_source_view</source_model>
        <sort_order>40</sort_order>
        <show_in_default>1</show_in_default>
    </view_style>
</fields>

create one file for multiselect option in your module in this path

your_namespace/yourmodel/Model/System/Config/Source/View.php

Add below code in your View.php

class YourNamespace_YourModule_Model_System_Config_Source_View 
{
    /**
     * Options getter
     *
     * @return array
     */
    public function toOptionArray()
    {
        return array(
            array('value' => 0, 'label' => Mage::helper('adminhtml')->__('Data1')),
            array('value' => 1, 'label' => Mage::helper('adminhtml')->__('Data2')),
            array('value' => 2, 'label' => Mage::helper('adminhtml')->__('Data3')),
        );
    }

    /**
     * Get options in "key-value" format
     *
     * @return array
     */
    public function toArray()
    {
        return array(
            0 => Mage::helper('adminhtml')->__('Data1'),
            1 => Mage::helper('adminhtml')->__('Data2'),
            3 => Mage::helper('adminhtml')->__('Data3'),
        );
    }
}

<default>
     <mytab>
        <mysection>
            <attributes><!-- ***WHAT SHOULD I WRITE HERE*** --></attributes>
        </mysection>
    </mytab>
</default>

You should use the comma separated keys of your options array.

eg

<default>
     <mytab>
        <mysection>
            <attributes>0,1,3</attributes>
        </mysection>
    </mytab>
</default>

Selects all three options by default.