Line Art
Just found a new tutorial that was posted on Jan 7th, 2013. This is quite a long tutorial. The name of it is "how to create a line art base symmetrical portrait in Adobe Illustrator" and the users are giving it a three out of five star rating. I believe the three out of five stars is due to the fact that this is a very lengthy and advanced tutorial.
The author starts off with the actual photograph of a young lady. He uses that photograph as a background and the base to which he will copy the right side of her face, and then duplicate that side, flip it and then paste the two halves together. The end result is quite amazing.
So if you're into Adobe Illustrator and would like to attempt a more advanced tutorial, then i would recommend giving this one a shot.