adding jQuery click events to dynamically added content

You want to use live events, which were introduced in 1.3.

$("tr.clickable").live("click", function() {
   //add input fields
});

$("span#addNewRow").live("click", function() {
   $("table").append('<tr><td class="clickable"></td> ... </tr>')
});

UPDATE: Note that as of jQuery 1.7, live() is deprecated. Use on() instead. And in some cases, delegate() may be a better choice. See the comments below.

Example of how to use .on():

$("table").on("click", "tr.clickable", function() {
   //add input fields
});