Formatting numeric amounts in Javascript

Dear Lazyweb,

Is there a better method than this to format numeric amounts? Here's a solution I picked up from somewhere and slightly modified. It's heavily string based but passed the tests:

function format_amount(i) {
  if(isNaN(i)) { i = 0.00; }
  var minus = '';
  if(i < 0) { minus = '-'; }
  i = Math.abs(i);
  i = parseInt((i + .005) * 100);
  i = i / 100;
  s = new String(i);
  if(s.indexOf('.') < 0) { s += '.00'; }
  if(s.indexOf('.') == (s.length - 2)) { s += '0'; }
  s = minus + s;
  return s;
}

The "tests" are:

format_amount(100)       == "100.00";
format_amount(100.0)     == "100.00";
format_amount(100.05)    == "100.05";
format_amount(100.051)   == "100.05";
format_amount(-100)      == "-100.00";
format_amount(-100.0)    == "-100.00";
format_amount(-123.45)   == "-123.45";
format_amount(-123.450)  == "-123.45";

So functionally it's OK but I'm not sure it's the best way to do it.