Yes, you are perfectly free to remove the RocketTheme copyright information from the footer. You must retain the following information in the template XML file:
<author>rhuk</author>
<authorEmail>rhuk@rockettheme.com</authorEmail>
<authorUrl>http://www.rockettheme.com </authorUrl>
<copyright>Copyright 2005-2006 - RocketTheme.com</copyright>