As per the kitchensink example, spacers control the positioning of buttons on a toolbar. In 1.1.1, the following code produces a button in the upper left, the title text centered, and a button in the upper-right:

Code:
   {
            xtype:'toolbar',
            docked:'top',
            title:'Title',
            items:[
                {
                    ui:'action',
                    text:'Left'
                },
                {
                    xtype:'spacer'
                },
                {
                    ui:'action',
                    text:'Right',
                }
            ]
        }
In PR3, the left button is in the upper left, the right button is in the middle of the screen, and the title text is shifted to the upper right of the toolbar.

Positioning of title text works correctly when only one button is used without a spacer; the button appears in the upper left and the title is centered as expected. I tried using {html:'Title'}, as a workaround between a set of spacers, but it seems that any text in between spacers throws all positioning off.