When create icon-only button ({text:false, icons{primary:"icon-gear"}}) button has class ui-button-text-icon-primary that move primary icon to the left side. If remove class icon moved to center.
upd: i see in jquery.ui.button.js line 296: append classes to array line 308: remove classes from element!!! buttonElement.removeClass( "ui-button-text-icons ui-button-text-icon-primary ui-button-text-icon-secondary" ); i think this need a fix. line 317: add classes from array => classes overwrited.