Providing the new Array being constructed via jQuery.grep() to the callback provided would allow inquiry into the new Array that could be useful when determining the return value.

https://github.com/jquery/jquery/blob/1.6.2/src/core.js#L722

Change:

retVal = !!callback( elems[ i ], i );

to:

       // added new Array as arg-----v
retVal = !!callback( elems[ i ], i, ret );

Possible usage example:

var arr = 
$.grep(
    $('div').map(function() { return $(this).attr('rel'); } ).get(),
    function( v, i, arr ) { return $.inArray( v, arr ) === -1; } // exclude dupes
);

**jsfiddle:** http://jsfiddle.net/b7hDr/

I would also be inclined to suggest this for jQuery.map():

var arr = 
$.map($('div'), function( v, i, arr ) { 
    var rel = $(v).attr('rel'); 
    return $.inArray( rel, arr ) === -1 ? rel : void 0;
});

**jsfiddle:** http://jsfiddle.net/b7hDr/1/

...although I don't know if it would interfere with the internal arg parameter.

https://github.com/jquery/jquery/blob/1.6.2/src/core.js#L742

     // make room for "ret"?------v
value = callback( elems[ i ], i, ret, arg );

https://github.com/jquery/jquery/blob/1.6.2/src/core.js#L752

     // make room for "ret"?----------v
value = callback( elems[ key ], key, ret, arg );