Adapted from http://thefinishedbox.com/freebies/scripts/jquery-dropdown-login/ Visit this site for more detail and alternate drop down box styles.
DEMO at http://www.alpha-clear.com/ninesixty/index.php
(click login at top right.)
If you are using OSC to CSS v2 (or v1) this is easy installation. No files or images to upload, just three easy steps. Download contains a text file with instructions and code to cut and paste.