I didnt' see this functionality anywhere so I wrote a plugin to accomplish it.
Example Usage: [popuponload height="200" width="200" title="Your Title Here"]<p>Pop-Up Content</p>[/popuponload]
The plugin uses the two shortcodes: [popuponload] [/popuponload]
Anything between these two fields will pop-up when that page is loaded.
Default hides the content and only displays it on pop-up but this can be changed with an option to allow the content to be displayed in the page.
More information and download link can be found here: