[muddi900]

In my jailbreak days, the DFU update did indeed wipe the device. I don't know how it works.

[bb88]

Did it actually write zeros to the flash memory or did it leave the data sitting around somewhere?

[leonatan]

The encryption key is deleted. All data can remain on the flash for years, but it's useless.

[mywittyname]

But if you're planning on brute forcing the encryption key, this side-effect doesn't matter.

[muddi900]

The key is not the password. If that were the case, the phone would have to re-encrypt everything everytime you change the pin or password. The password unlocks the key. And if you brute force the key itself, it might take decades, maybe centuries.