There is an option under Admin > Permissions to set "Viewing question pages" to registered users only. This will allow any visitor to view the list of questions, but only registered users the full question, and answers.
If you would like to allow unregistered users to view the question text, then you would need to make some modifications with a plugin, which I don't think would be too difficult.
One thing you could do is modify the "mouseover layer" plugin - which shows the start of the question text in the question list - to display that right on the page.
