I found this vid through
amy_vic and really think everyone should see it
http://www.campaignforrealbeauty.com/home_films_evolution_v2.swf It shows the process of prepping a model for a billboard shoot and then the crazy amount of photoshopping that gets done in the end! I honestly never thought that much was done, just the basic airbrush and you're good to go.
And when you're done that and need a good giggle:
Wicked+SheRa. Seriously, it's brilliant. A huge thanks to
thebitterguy for sharing that link :)