"Install" or "upgrade" function should add <class> to <setup> in config.xml.
Like
<setup>
<module>Namespace_Module</module>
<connection>core_setup</connection>
<class>Mage_Sales_Model_Mysql4_Setup</class>
</setup>
I think it's not very good idea since we don't know which class should be used in all cases, so without Magento will use default setup class and users may change it manually to the specific one if needed.
"Install" or "upgrade" function should add <class> to <setup> in config.xml. Like <setup> <module>Namespace_Module</module> <connection>core_setup</connection> <class>Mage_Sales_Model_Mysql4_Setup</class> </setup>