Is it possible to create a single-user USB installation (with persistence) of Ubuntu Linux such that the entire USB stick is encrypted and requires a passphrase at boot time?
Is there an online tutorial for this?
It should be straightforward to make a persistent installation directly on a USB stick, as if it was an internal disk. Plug in your Ubuntu installation media (I recommend not putting it on the same stick, so that the two are bootable separately), your USB stick, and point the installer to the stick.
The server installer (alternate CD) supports creating and installing to an encrypted partition (with dm-crypt).