Petter Silfver on the tab bar in iOS:

Since the screen width of an iPhone is 320 dots (1 dots = 1 or 2 pixels), Apple designed the standard tab bar to contain no more then five tabs to be able to maintain visible iconography and readable copy.

Silfver brings up a ton of good points relating to the tab bar — a great read for developers or people nerdy enough to be interested in tab bar design (me).

Posted by Ben Brooks