In Magento, if you’re using a pull-down attribute that needs to be set to default instead of the “Choose option…” default automatically provided by Magento, you are theoretically supposed to be able to set it from within the Admin portal by using Catalog > Attributes > Manage Attributes. Unfortunately, it looks like there is a bug that isn’t allowing you to do that.
To fix this, open up the JavaScript file /js/varien/product.js and change line 356 from var index=1; to var index=0;
This will force Magento to pay notice to your preferred default in the pull-down rather than displaying “Choose option…” and forcing the user to select something.
Caveat: If you are using more than one custom pull-down/drop-down attribute, you’ll probably screw some stuff up.