I have created a new plugin, WP-ShowHide. It is a simple plugin (83 lines of codes including comments) with no configuration screen needed.
It allows you to embed content within your blog post via WordPress ShortCode API and toggling the visibility of the cotent via a link. By default the content is hidden and user will have to click on the “Show Content” link to toggle it. Similar to what Engadget is doing for their press releases.
[showhide type="pressrelease"]Press Release goes in here.[/showhide]
I tested it on WordPress 3.1.1 but technically it should work with any version higher than WordPress 2.5 because ShortCode was introduced in WordPress 2.5.
WP Plugins Page: http://lesterchan.net/portfolio/programming/php/#wp-showhide