The idea that first came to mind was to use CSS3's :target pseudo class. We talked about these a while back while exporting CSS3 tabs. The idea is to have a link that links to the #source-code hash tag. That puts #source-code at the end of the URL, and allows the :target selector to match and apply styling.
CSS-Tricks/View-Source-Button
Using jQuery, Code Prettify, and CSS3 :target, we can make a button that pops up a nice looking view of the current pages source code.
JavaScript