Do what some of the guys around do. Sending a launch mailing to your list after you've let them know it's coming, but have the link take them to a page that says it's sold out(don't worry you will sell to them a day or 2 later).

This can actually get the person talking about it at work or to friends or family(when have you not known a disappointing moment in someones life not to be drawn out into a story?)

Picture the customer saying something like "I went to buy this product, guide or course and it was sold out" to someone and of course the conversation begins when the person their talking to says "what the hell's that?"

After this just send a blast that says you've "re-opened" the download page due to "high ass demand" or whatever you wish to say ;D