Actually, I stated it wrong. For data that has been encrypted it only erases the encryption key... not necessarily overwriting the data with zeroes.
That data can be retrieved by a diligent enough person.
He asked about securely erasing the phone. I'm sure it's secure enough for most peeps.
iPhone 3Gs and later uses the "throw away the encryption key" method.
Here's the Apple kb article.http://support.apple.com/en-us/HT201274
Never is a veeeery long time.