Sexy Persistent Header with Opacity Using jQuery

I’ve been working with the Magento eCommerce solution a lot lately and I’ve taken a liking to a technique they use with the top bar within their administrative control panel. When the user scrolls below a specified threshold, the top bar becomes attached to the top of the window and the opacity set to 50%. I’ve implemented this technique in my current design and have gotten numerous requests for a tutorial so…here you go!