Question: I recently installed Ubuntu 12.10 and it requires a passphrase to boot up (I installed it with an encrypted file system).

Do I have to reinstall to change to a standard unencrypted file system?

Answer: 
In case it is OK to keep the encryption, but to switch off the passphrase prompt, a much simpler approach is to just set a trivial password like "password" and then save that trivial password in the initramfs in cleartext. [Disable the LUKS encryption password](http://atterer.org/linux-remove-disable-luks-encryption-password-on-disk-partition-crypttab-initrd).

Essentially, add a hook script which in turn adds a "keyscript" to the initramfs. Usually these scripts are used to get the password via Bletooth, from a USB stick etc., but in this case, just make it print the trivial password.
@pattivacek 's short answer worked fine for me.
To get rid of the start screen "Enter your password" part - i just added

```
apt purge cryptsetup

```

after that.