In your config.xml you could also do
config.xml:
<global>
<models>
<mymodulename>
<class>MyNamespace_MyModuleName_Model</class>
</mymodulename>
</models>
<events>
<controller_action_predispatch_customer_account_createpost>
<observers>
<mymodulename>
<type>singleton</type>
<class>mymodulename/observer</class>
<method>hookToAccountCreationBefore</method>
</mymodulename>
</observers>
</controller_action_predispatch_customer_account_createpost>
</events>
</global>
or
<global>
<events>
<controller_action_predispatch_customer_account_createpost>
<observers>
<mymodulename>
<type>singleton</type>
<class>MyNamespace_MyModuleName_Model_Observer</class>
<method>hookToAccountCreationBefore</method>
</mymodulename>
</observers>
</controller_action_predispatch_customer_account_createpost>
</events>
</global>