the whole point of having it notarised, is that it proves that you are who you say you are... so if it is signed before, or not notarised you could technically have it signed by a friend or even sign it yourself. their idea in requesting such a letter is to stop kidnapping of children by divorcing parents, i believe.
i don't remember if i read about signing it on the website, but we've had a letter notarised twice and that was the procedure. you write the letter, take it to the office, sign it in front of the officer/person, they notarise it... done
however, the first time we did it, it wasn't notarised, i typed a letter out on the computer and had hubby sign it. the immig officer gave me a hard time at canadian immig and suggested that the next time it be notarised, if i remember correctly. it was almost 4 years ago, though, so i don't remember exactly.