I don't do many illustrations, so I'm uncertain about this one. Basically, I traced Jessica Alba in Flash. Will I need a model release for the illustration? And if I do need a model release, how do I go about getting Jessica to sign one? 

If you traced someone else's photograph of her then istock for one won't accept it - you need to be able to provide your sources (and it's really not quite the done thing to use someone else's photograph as a trace and then sell the result without getting the photographer's permission.) I thought the face was vaguely recognisable - and if you are submitting somewhere that they like to see the source then the chances are you will need a release

Whiz, Susan S is correct, there was another person on this site a while ago who got banned because when they submitted, they forgot to delete the layer they traced from.

Personally, I don't see an issue if:

You only use the pic as a source for reference points. Similar to what an artist does when painting.
The image is not "photographic" (ie) the image is cartoonish etc.

Remember, artists, all artists (including photographers) tend to be a little bit precious about their work.

RE: Bateleur ... the pic here is not Jessica's best side