Even before yesterday's official presentation of the new device, a mass mailing campaign offering a protective case for it has been spotted by Kaspersky Lab researchers:
Now - even if this offer was legitimate, it is highly unlikely that the case would fit, as the iPhone 5 is thinner and longer than its predecessor. The fact that the senders sent out the email before the release of the device indicates that this is likely a scam.
It's hard to tell just what type of scam it is, but at best you can get saddled with a case that doesn't fit, and at worst your credit card information can be stolen and used by the scammers.
In any case, beware of offers like these and restrict your online shopping to legitimate e-commerce sites.