To test if a checkbox has been ticked is a bit tricky.
Using val() will return the potential value of the checkbox, regardless of its check state.
Instead, use the following line to test its state:
$("#checkbox").attr('checked') This will return true or false depending on its checked state.
*update 27/07/2011*
Use the new .prop() function:
$(".myCheckbox").prop("checked", true);
$(".myCheckbox").prop("checked", false); jQuery 1.5 and below
The .prop() function is not available so you need to use .attr().
To tick the checkbox (by setting the value of the checked attribute)
$('.myCheckbox').attr('checked', 'checked') and un-checking (by removing the attribute entirely)
$('.myCheckbox').removeAttr('checked') 