Once you register your mobile number via Telstra's Facebook presence (linked below) and confirm via a pin sent to you as a text message, you can leave a voice update of up to 30 seconds by ringing 0458 258 785 and dictating your update. You can also nominate up to five friends in order to leave voice messages on their Facebook wall. (Sadly, that only works for actual people on Facebook, so I wasn't able to leave a Blurtl message on Lifehacker's Facebook page.)
The service worked pretty seamlessly when I tested it, though the audio quality isn't spectacular. There's also no way to customise the appearance of the message on your wall — if you leave four voice updates during the day, they'll all be identically tagged as 'New Blurtl message', with only the time stamp to distinguish them.
According to Telstra, the service should work with any Australian mobile, regardless of your network. It's a free service (outside of call charges), though you have to put up with brief Telstra audio ads before you can leave your message.