How would you do this touchup?

http://www.modelmayhem​.com/1664586

That's the link for the picture.

That sort of stuff would be done mainly with the clone and stamp tools.

Relatively simple, assuming adequate software and PP skills.

Skin smoothing can be a little time consuming. Now you know the phrase "skin smoothing photoshop" you can google it to learn how

Not difficult, but time-consuming if you want to do it right. (although that's relative. I'd say under an hour)
Frequency separation. And local dodge / burn / sharpen (eyes). He also brightened or replaced the background, which could have been done a bit better IMO.

Here ya go.

The Perfect Lie

Google the name "Natalia Taffarel." She specializes in this kind of retouching and she has tutorials on DeviantArt and Model Mayhem. She also has a DVD where she demonstrates her techniques. As already pointed out it is extremely time consuming: Beauty & Hair Retouching: High End Techniques