I sold a friends iPhone 4 on eBay for him. He had around 15GB on there and when he went to the post office to mail it he did the "erase all content and settings". He said it took literally 10 seconds. He was concerned the data was not deleted so I had him restart and activate the phone to check. Everything was gone. He erased all content and settings again but is not convinced. I guess he has some proprietary work information on there and is concerned the person it is being sold to will attempt some kind of data recovery. My question is- given that this is flash memory, is it recoverable?