There are two scripts that do what you want.
1) Here's a Script. It works as good as the track you get:
http://del.icio.us/escruz/Rotoscope
Download, then copy/move the jsx file RotAE.jsx to AE's Script Folder, then open AE.
Select the layer with the Mask, File>Run Script>RotAE.jsx. Then select the Mask again, and in the control panel that popped up, hit "Apply Tracker", and you'll have as many track points as you have mask vertices.
Run you Motion Track. The mask will be as good as the motion track so you may have to adjust points by frame like you mentioned in Aharon's tut.
Now twirl down the layer, select the Tracker 1 (above the mask points) and also select the Mask, then up in the Script's Control panel, click " Apply Mask". You should be good to go.
You can still move individual mask points if the track wasn't great, or adjust the bezier handles as needed, as the script just set mask Shape keyframes at each frame.
2) There is another script, "rotoTool", I haven't really used it yet, but his stuff is great. Here's the script:
http://www.nabscripts.com/Downloads/downloads_en_3.php?id=1
His site is French and some of the scripts a low on translation, but the Help "?" in the script panel for this particular script is English, and looks very straightforward, yet versatile. It converts tracks to masks, or reverses that converting masks to tracks.
Lots of other interesting scripts there too.
