If you are migrating from a single instance of Drupal to a multi-site installation, you may encounter a problem or two along the way in terms of certain things breaking as they aren’t where they used to be. Since Drupal is almost solely reliant upon the database during it’s bootstrap process, more often than not if something is misconfigured you can end up with the white screen of death (which neither PHP or Apache will be able to pick up)
Theme fixing… (http://drupal.org/node/200774)
UPDATE system SET status=1 WHERE name = ‘garland’;
UPDATE variable SET value=’s:7:”garland”‘ WHERE name = ‘theme_default’;
TRUNCATE cache;
Files fixing… (http://flevour.net/blog/drupal-changing-files-directory-configuration-setting)
UPDATE `files` SET `filepath` = REPLACE(`filepath`, “files/”, “sites/SITEDOMAIN/files/”);