Automatically have link back to main blog on subdomain blogs?

Is there a way to set Wordpress MU up so that every subdomain blog will automatically have a link back to the main blog?

I notice that there is automatically a tagline "just another TPLC Times weblog" and I tried editing that in wp-admin/includes/schema.php but it has appeared to make no difference (it's still showing the default - do I need to re-start something in order for it to "take"?)

At any rate, all I did was change it to "A %s weblog" because I had no idea how to make the "%s" into a link back to the main blog, which would be ideal.

If anyone has any suggestions on how I can accomplish this, let me know.

Thanks,
Yvonne