How to securely erase/format an iPhone for resale? I've read a couple guides that talk about restoring the iPhone to it's base operating system, then syncing three different full playlists of music in order to overwrite any previous personal data that might be on the phone and recoverable via basic forensic methods.

I want to believe there is a better way, maybe a simple program that allows you to write zero byte files until the OS is full a couple times over.

What is the best way to format/erase an iPhone for resale?
The built in “Erase all Content and Settings” function (found in the Settings app, under General → Reset) should be sufficient in virtually all cases.

The 3GS and later iPhones, 3rd generation and later iPod Touches and all iPads use hardware encryption, where all data is stored encrypted in their flash memory. On these devices the wipe function simply deletes the encryption key, rendering the data not much more useful than random noise. On devices without hardware encryption, the wipe function overwrites the entire user partition with ones. This is more secure with flash memory than doing the same on a hard drive (where some residual magnetic field can be detected from the previous data, although the chances of that are likely very low) — it's theoretically possible someone could retrieve some data, but it would be prohibitively expensive and time consuming.

Unless you're keeping extremely sensitive data on there (i.e. state secrets or evidence of a massive criminal conspiracy), I wouldn't worry about it once it's wiped.

Source: Apple Article HT2110 — iOS: Understanding 'Erase All Content and Settings'