To change this in the standard Joomla login module,
Open modules/mod_login/tmpl/default.php and on or near line 66, replace the link so you end up with this:
<a href="<?php echo JRoute::_( 'index.php?option=com_community&view=register' ); ?>">
<?php echo JText::_('REGISTER'); ?></a>
There is one other place where this link shows up.
In components/com_user/views/login/tmpl/default_login.php replace line 64 with the following:
<a href="<?php echo JRoute::_( 'index.php?option=com_community&view=register' ); ?>">
This was done in Joomla 1.5.8 for JomSocial 1.0.27