Also, the Checkout Buttons for Mt. Gox have a checkbox where you can indicate that you would like autosell (to have the bitcoins sold at market to USDs). Here's a site using that -- see the "site-wide pass" at the bottom:
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http://chipcollection.com/webstore/ordering.htmlThat's cool that as you hover over the button it updates price as market changes. Now if I could just think of a great idea that hasn't been done already...