From: Angus Mezick <amezick@edgil.com>

Have an action that could be remapped to the tab key and does the following:

re-formats the current line.
if line is blank,  
  move the insertion point to the correctly indented location.
else
  re-formats the current line.
  if the insertion point is in the indented area (before the first 
     character on the line)
    move the insertion point to the first character of the line.

From emacs c-mode:
c-tab-always-indent's value is t

Documentation:
*Controls the operation of the TAB key.
If t, hitting TAB always just indents the current line.  If nil,
hitting TAB indents the current line if point is at the left margin or
in the line's indentation, otherwise it insert a real tab character.
If other than nil or t, then tab is inserted only within literals
-- defined as comments and strings -- and inside preprocessor
directives, but line is always reindented.

I like to define this as 't' so that I can indent anytime anywhere and not
care about how many spaces or how many levels.  This is very useful to
double check to make sure that you aren't missing a paren, brace, or
bracket.