I tried this: http://jsfiddle.net/ilyaD/KGcC3/
HTML:
I'm afraid this isn't possible with plain CSS, and won't be possible to make completely cross-browser compatible.
However, using a jQuery plugin, you could style the dropdown:
https://www.filamentgroup.com/lab/jquery-ui-selectmenu-an-aria-accessible-plugin-for-styling-a-html-select.html
This plugin hides the select element, and creates span elements etc on the fly to display a custom drop down list style. I'm quite confident you'd be able to change the styles on the spans etc to center align the items.