Is there a tool I can use to securely wipe data from iPhone, iPad, or iPod Touch? Is there a tool or application that will securely wipe an iPhone, iPod Touch, or iPad (i.e. any iOS device?)

I'm not interested in a complete wipe that would brick the device.  Rather, I'd want to wipe all free space after it's been reset to default or had the OS reinstalled.

Furthermore: by "securely wipe", I'm referring to multiple successive passes with random data, etc. to thwart any attempt at data recovery.

For instance: If somebody wanted to sell their old iPhone on eBay, how can they be sure it's completely wiped, so there's little hope of a malicious buyer being able to recover sensitive data that used to be on it?

I think this is especially important when people are using iPhone apps to store passwords, credit card numbers, etc. (even if those apps have a supposedly secure storage mechanism.)
iOS 4.0 supports a very rapid secure erase called "Data Protection". If you upgrade from 3.0 you need to do a restore to set the stage for hardware encryption of all the data.

If your device isn't capable of hardware encryption (often called secure data at rest), then you can use the Erase all content and settings function. This built-in functionality to secure wipe your iDevice has been around since iPhone OS 2.0

As this doesn't work the same way on all iOS devices, do read Understanding 'Erase All Content and Settings' before you depend on this to clear data securely.

Once you are through erasing the device, you might want to connect it to a clean iTunes library and restore the device and set it up as new. Making a new account on your Mac or PC is an easy way to have a library with no account, no songs, and no apps just to be sure the device has a clean (and easily verifiable) start to it's new life.