Meet Genius Mixes. This new feature searches your iTunes library, finds songs that go great together, and creates multiple mixes you’ll love. All automatically. These mixes are like channels programmed entirely with your music. You may discover songs you never knew you had — and rediscover forgotten favorites.

But after upgrading to iTunes 9, I didn’t see the Genius Mixes in my playlist bar. I found the (now obvious) answer over on MacOSXHints.com:

If you update to iTunes 9 and don’t see the Genius Mixes (even though you have the Genius enabled), simply select Store » Update Genius.

Once the Genius update is complete, the Genius Mixes entry should show in the sidebar.

I’m comforted to note that even Rob Griffiths — THE Mac expert behind OSXHints.com — was stumped by this one as well.