WordPress MU

I'm trying to setup my blogs so that it's only by numbers how do I do that?

ie: http://www.test.com/12121/

Wordpress mu wouldn't even let me use only numbers to register.

Take a look at wp-includes/wpmu-functions.php

Line 1014 for this check and comment it out or delete that section.

// all numeric?
preg_match( '/[0-9]*/', $blog_id, $match );
if ( $match[0] == $blog_id )
$errors->add('blog_id', __("Sorry, blog names must have letters too!"));

Hope this helps,
-drmike