1) small one: provide a rendering for design mode so just a small button and not the large control shows up
2) provide an additional property: default style true/false. If set to true it uses the nice formatting from your demo (you actually could call it "Lazyman's format)
3) Supercool: a complementary control that shows the selected values with a little x behind to deselect. Like this one:
http://designingwebinterfaces.com/the-multi-select-solution (not the dropdown part - I like yours better, but the results part)
:-) stw