In 1.8version if we go to "Manage Customers" from the admin section, we can do search on customers by "Date Of Birth" BUT if we upgrade from 1.7 to 1.8version, admin doesn't get the option to "Date of Birth". So i guess something is missing in upgrade script.
2nd Bug)
Search by "UserName" in missing in 1.8version "fresh install" - In the Admin section > manage customers