A widget for displaying price using that is easy. Getting your shopping cart software to display and use that price using that is a little more complicated, requiring customization likely at this point.
As far as a widget -- here's a thread on one:
http://forum.bitcoin.org/index.php?topic=8895.0
Perhaps either of the widget authors from the thread will consider adding a configurable weighted average.
[Edit: a json feed with weighted average prices (1-day, 7-day, 30-day) here: http://bitcoincharts.com/t/weighted_prices.json ]
Thanks, this is just what I was looking for.