Re: What is the name of the flap of skin between the thumb and first finger?

The name of the flap of skin is the "first web."

This is from Gray's Anatomy:

The proximal segments of the fingers are joined to one another on the volar aspect by folds of skin constituting the “web” of the fingers; these folds extend across about the level of the centers of the proximal phalanges and their free margins are continuous with the transverse furrows at the roots of the fingers. Since the web is confined to the volar aspect the fingers appear shorter when viewed from in front than from behind.

hope this helps
gabriel vargas md/phd

references
Gray's Anatomy
 http://www.ccmbel.org/Anatomy.html
 
 http://education.yahoo.com/reference/gray/subjects/subject?id=289#2