If you used
jQuery.fn.exists = function(){return ($(this).length > 0);}
if ($(selector).exists()) { }
you would imply that chaining was possible when it is not.
This would be better:
jQuery.exists = function(selector) {return ($(selector).length > 0);}
if ($.exists(selector)) { }
Alternatively, from the FAQ:
if ( $('#myDiv').length ) { //* Do something */ }
You could also use the following. If there are no values in the jQuery object array then getting the first item in the array would return undefined.
if ( $('#myDiv')[0] ) { //* Do something */ }