Here’s one that works (see the example):
function updateTextArea() {
var allVals = [];
$('#c_b :checked').each(function() {
allVals.push($(this).val());
});
$('#t').val(allVals);
}
$(function() {
$('#c_b input').click(updateTextArea);
updateTextArea();
});
Update
Some number of months later another question was asked in regards to how to keep the above working if the ID changes. Well, the solution boils down to mapping the updateTextArea function into something generic that uses CSS classes, and to use the live function to monitor the DOM for those changes.