http://www.raspberrypi.org/phpBB3/viewtopic.php?f=36&t=47029
It's a similar process to setting up Netatalk on any Ubuntu server to get Apple computers to recognize a Linux server as a Time Machine endpoint.
An actual Time Capsule can be annoyingly slow sometimes, and I would expect it to be about five to ten times as fast as a Pi-based solution.
The link you posted has some good advice though. Like not using HFS+. Apart from HFS+ arguably being an antiquated filesystem, the Linux driver is in a horrible state and will cause corruption when used in rw mode. ext4 is the sensible choice.