My answer will be in the form of links.
The WPAlchemy meta box class supports this: http://www.farinspace.com/wpalchemy-metabox/
+ tutorial http://www.farinspace.com/wordpress-media-uploader-integration/
A tutorial on how to do it, mainly javascript: http://www.webmaster-source.com/2010/01/08/using-the-wordpress-uploader-in-your-plugin-or-theme/