Disable submit button unless original form data has changed

You just need to save the orignal values and then check against them after every change.

Example: http://jsfiddle.net/justincook/42EYq/15/

JavaScript:

// Store original values
var orig = [];             
$("form :input").each(function () {
    var type = $(this).getType();
    var tmp = {'type': type, 'value': $(this).val()};
    if (type == 'radio') { tmp.checked = $(this).is(':checked'); }
    orig[$(this).attr('id')] = tmp;
});

// Check values on change
$('form').bind('change keyup', function () {
    var disable = true;
    $("form :input").each(function () {
        var type = $(this).getType();
        var id = $(this).attr('id');    
        if (type == 'text' || type == 'select') {
            disable = (orig[id].value == $(this).val());
        } else if (type == 'radio') {
            disable = (orig[id].checked == $(this).is(':checked'));
        }    
        if (!disable) { return false; } // break out of loop
    });

    $('#submit-data').prop('disabled', disable); // update button
});

// Get form element type
$.fn.getType = function () { 
    if(this[0].tagName == "INPUT")
        return $(this[0]).attr("type").toLowerCase() 
    else
        return this[0].tagName.toLowerCase();        
}

HTML:

<form id="" action="" method="post">
    <input type="text" value="Carl" name="firstname" id="firstname" />
    <input type="text" value="Johnson" name="lastname" id="lasttname" />
    <input id="radio_1" type="radio" value="female" name="sex" />Female
    <input id="radio_2" type="radio" value="male" name="sex" />Male
    <select id="country-list" name="countries">
        <option value="xx" selected="">Worldwide</option>
        <option value="in">India</option>
        <option value="us">United States</option>
    </select>
    <button id="submit-data" disabled="" type="submit">Save changes</button>
</form>

An improved version of @Nigel Angel answer:

/* Disable the forms submissions until they have changed */
$(window).on('load', function (e) {
    $('form').each(function(){
        $(this).data('serialized', $(this).serialize())
    })
    .on('change input', function(){
        $(this).find(':submit, input[type="image"]')
            .prop('disabled', $(this).serialize() == $(this).data('serialized'));

        /* Check if input with type files has changed */
        var changedFiles = $( ":file" ).filter(function( index ) {
            return this.value != this.defaultValue;
        }).length;

        if ( changedFiles > 0) {
            $(this).find(':submit, input[type="image"]')
                .prop('disabled', false);
        }
    })
    .find(':submit, input[type="image"]').prop('disabled', true);
});

• Added the input[type="image"] to the list of selectors that perform submission.
• Added a check for changes of input with type file.
• Also shortened the answer to use :submit to cover both cases of button and input.

I know this is a late answer, but this method uses far less code than the accepted answer without needing IDs on fields and without storing a global array.

Simply use .serialize() to store the data and check if it matches on each change.

http://jsfiddle.net/Pug3H/2/

$('form')
    .each(function(){
        $(this).data('serialized', $(this).serialize())
    })
    .on('change input', function(){
        $(this)             
            .find('input:submit, button:submit')
                .prop('disabled', $(this).serialize() == $(this).data('serialized'))
        ;
     })
    .find('input:submit, button:submit')
        .prop('disabled', true)
;