Check out this [WordPress Q&A Plugin](http://premium.wpmudev.org/project/qa-wordpress-questions-and-answers-plugin) by [WPMU Dev](http://wpmudev.org). They have completely ripped off the Stack Overflow design in their plugin, even down to the details like the accepting answers tick and tabs.

What's worse is that they are charging $79 a month to have access to this plugin, essentially profiting by copying the SO design (and concept, but I understand you couldn't copyright that).

Just to make this a question: Is this allowed/legal?