Ok, not quite what I was looking for but not bad for a start.
Headshot Photography Basics
I'm not sure why this article doesn't talk of the horizontal headshots but it could just be that they think nobody's going to fall for this...
Some photographers are shooting horizontal headshots these days but, using plain old common sense tells us this is a no-no. Your resume (attached to the headshot) is vertical as are all the other ones in the pile being reviewed. When the casting director gets to yours, he/she has to turn things around to view your photo. More than likely, it will not get turned and you will not get the job.
If somebody asks you to shoot horizontal, just tell them why it is not a good idea. If they insist, make sure you have a clause in your contract that they can initialize stating the horizontal was their decision.
Headshots can be done outdoors so long as the background is totally oof and very plain. Actors headshots can be a bit more dramatic with the lighting but not models. As the article says, you are showing the person as they are. Get called in for an interview and your headshot doesn't look like you and you will never work for this person.
Hope this gets you started.
Good luck.
Edit = Btw you don't need a fancy studio to do headshots. You can do them with minimal, cheap gear including homemade reflectors and there are plenty of tutorials for that on the web.