Wipe that old iPhone with 2.0 before you eBay!

I had heard through the security grapevine months ago that it was easy-peasy to recover old iPhone data from used and refurbished iPhones, even if data was "deleted". Apple finally got the word, and has included the ability to wipe everything - just go to Settings > General > Reset > Erase All Contents and Settings. You'll need to upgrade to version 2.0 in order to do so.

Still, it probably wouldn't hurt to overwrite using this method, either.