Smart Playlists in iTunes 9 can now be more than just (A and B) or (A or B or C) to build a playlist. Now the rules can be combined and nested. In addition to the plus and minus buttons, there's a new three-dot (ellipsis-like) button to generate the nested rules. For example, A and (B or C) was previously not possible.
In iTunes 9, you can now do this. First add rule A and set the top "Match" criteria to All. After you have rule A done, click on the three-dot button to get a lower level rule, add rule B and set its applicability pop-up to Any. Finally, click the plus sign and add rule C. This will create the logical structure of A and (B or C). You can get even more fancy and click the three-dot button again, and add a D or E conditional, giving you a final rule structure of A and (B or C and (D or E)).
The iTunes 9 help is silent about this new feature, but it's quite powerful.
Mac OS X Hints
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