I added one feature, and made another minor change.

• DisplayMode attribute: horizontal or vertical
  This attribtue will flip the two select boxes to be side by side, or stacked on top of each other.
  Added CSS selectors for horizontal versus vertical mode for all elements. This lets you control the visual styles separately.
• Removed CSS style rules from the custom tag
  I pulled the CSS rules out of the custom tag, and moved them to an external file. The custom tag does not link to this file, the template consuming the tag handles that. The downside is that the tag may not look perfect out of the box, but the upside is that you can put the rules where ever you keep the rest of your site style rules.

Check out the screen shots to see an examples of both display modes.

I just posted a first release of code the select swap custom tag. It presents two select boxes, one with available options and a second with selected options. Users can move items between the two boxes by clicking on buttons.

I've tested it out in the latest versions of Firefox, IE, and Safari. But I'm sure there's going to be some browsers that it breaks in. Please post if you have any trouble.

The tag adds a couple form fields to a template. One is named after the id that you provide. This field holds the selected values in list form. The other is the #id#_display. This field holds the selected items' display text.

When JS is disabled, the output degrades down to a single select box with items selected.

The output look/feel is completely configurable with CSS.