I think in you just browse the sites of photographers who advertise for headshots in your area that you'll see a range of what is typical. Here is a site with lots of headshots (in the LA area):
http://studiomark.com/headshots.html
And here is a simple discussion of the subject:
http://www.yourtype.com/survive/headshots.htm
And here are examples galore from a head shot photographer (scroll down a bit):
http://www.boblasky.com/headshot.html
My research seems to suggest that you shoot the whole range (see the above link) and then provide the actor or subject with their choice (and they will likely pick some of each). In other words don't limit your shooting to what you think the market at a given time wants, shoot the range and provide it all. In the age of digital and digital proofing it only takes a few seconds to shoot the full range of any given pose.
Re: What are the current trends in head shots?
Cheers,
rfs