iTunes has always filled the iPod shuffle with music to capacity without asking for much intervention from you, but Apple snuck the same very handy functionality for all iPod models in the recent iTunes 8.1 release.
If you’d like to enable AutoFill with your iPod or iPhone, Ars has the details:
Visit the Summary tab and make sure you’re set up to “Manually manage music and videos.” Once you’ve done that, use the triangle to reveal the contents of your iPod or iPhone, and click on Music, at which point you’ll see the source selector, Autofill settings, and Autofill button at the bottom of the screen.
We did mention this in the bullet-list of features that were added to 8.1, but if you missed this useful addition, it’s worth highlighting. In fact, while iTunes DJ and Genius for TV and movies were the marquee features in the latest release, we’d argue that in terms of actually useful functionality, AutoFill is probably the best addition to iTunes 8.1 for iPod owners.