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).
I could not figure out how to make the mt. gox checkout button dynamic... When creating the button you have to put in a value... even if you change that value in the code they give you, once the customer clicks the button and goes to mtgox site the original amount shows back up again. This makes the button useless for shopping carts with different values at checkout.