A: 

I've found what seems to work by a little bit of trial and error. I would still be interested in a better comprehensive solution (and/or I need to point this out to the ThemeRoller team).

I made this change to the jquery-ui-themeroller.css:

.ui-dialog-buttonpane {
    position: absolute;
    bottom: 0;
    left:0;  /* Added this line and it makes the button pane anchor to the left */
    width: 100%;
    text-align: left;
    border-top: 1px solid #707c5a;
    background: #faf7eb;
}

I'm still doing some testing, but it seems to work on IE6/7 and FF2/3.

CMPalmer