Not so sure about that. SPF 45 means that 45 mins slathered up in it in the sun is equivalent to 1 minute not slathered up in it. I don't know any other interpretations, and this one is from the American Cancer Society.
If that wasn't the case, I think I'd have seen the lawsuit by now.
Regardless, slap it on, wear a hat, drink plenty of water and stay in the shade as much as possible. I'm a big fan of the Neutrogena Ultra Sheer as even at high SPFs you don't end up sweating like a fat lass.