"We can't search for anything if this is empty :-(" It's not empty though... it says "chad fowler". I understand this is just a placeholder, but for those who aren't developers and may not appreciate the difference, clicking search when the input field is empty should perform a search for the placeholder text.
Huh, never considered this before. If I had a placeholder username/password on my login box, should just clicking "login" take the user to a demo/guest account, or should it bounce them and tell them to fill it in? What would be more expected there?
Another example might be a flight search service where the placeholders for "Departing" and "Arriving" contain "Los Angeles" and "New York" plus valid leave/return dates. If you're trying to show people (who may not be actively looking for a flight at the time) how awesome your service is, clicking on the "Show Flights" button should use those values because the whole point is to demonstrate your service's unique selling proposition.
For a login/demo where your goal is to get people to try the demo and convert into a signup, I suspect you will find it more effective to offer trying a demo account via an obvious, in-your-face call to action rather than dual-purposing your customer login form.