jQuery appending an array of elements

You could use an empty jQuery object instead of an array:

var elements = $();
for(x = 0; x < 1000; x++) {
    elements = elements.add('<div>'+x+'</div>');
    // or 
    // var element = $('<div>'+x+'</div>');
    // elements = elements.add(element);
}
$('body').append(elements);

This might be useful if you want to do stuff with newly generated element inside the loop. But note that this will create a huge internal stack of elements (inside the jQuery object).

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It seems though that your code works perfectly fine with jQuery 1.8.

You could just call

$('body').append(elements.join(''));

Or you can just create a large string in the first place.

var elements = '';
for(x = 0; x < 1000; x++) {
    elements = elements + '<div>'+x+'</div>';
}
$(document.body).append(elements);

Like you mentioned, probably the most "correct" way is the usage of a DocFrag. This could look like

var elements = document.createDocumentFragment(),
    newDiv;
for(x = 0; x < 1000; x++) {
    newDiv = document.createElement('div');
    newDiv.textContent = x;
    elements.append( newDiv );
}
$(document.body).append(elements);

^{.textContent is not supported by IE<9 and would need an conditional check to use .innerText or .text instead.}