I can’t replicate your issue when using Twenty Ten.
Well, i dont changed theme for test installation, it’s just default wp coming from download link on the front page on wordpress.org
Can you say what browser you are using, if possible?
Hmm, it is impossible if you have not updated jquery ui draggable to 1.8.11 by hand
Found a root of the problem, if someone interested
It is denoted here:
http://jqueryui.com/docs/Changelog/1.8.11
http://dev.jqueryui.com/ticket/5563
And wordpress 3.1.2 have jquery.ui at 1.8.9 version by default