I simply cannot get myself to see this icon as a regular pentagon! I often select the 'diamond' (canted square) icon by mistake.
This is an interesting shape, though. It looks like it's the bottom half of a regular hexagon with half a square stuck on top of it. Does this shape have any interesting properties? What is the ratio of long edge:short edge? If the short edge is length 1, what are the lengths of the diagonals? These lengths and areas may not play well together; the hexagon is related to √3 while the square is related to √2. It might be fun to investigate anyways. The angles of this shape are 120°, 120°, 90°, 105° and 105°. And the average of these angle is... exactly 108°! The same as the angle measure of a regular pentagon! (Is this a surprise? Should it be a surprise?)
All right, all fun aside, the icon confuses me when I want a regular polygon (which happens more often that you'd think). I submitted feedback to Apple today asking them to change this icon... I'm just doing my job to promote good geometry. Do you use Apple Pages to make drawings? Has this bothered you? Here, tell Apple: http://www.apple.com/feedback/pages.html.