Here is a sample http://jsfiddle.net/mUpjw/15/
I had JQuery 1.2 in some old code and it was opening dialog this way.
$('#myDiv').dialog('open');
I upgraded to jquery 1.6.1 and it was working fine . But if I add a DOCTYPE than it doesnt work but if I do
$('#myDiv').dialog();
That works fine.
What can be reason for this ?
You need to setup your dialog box.