As you may be aware, by using the Command key modifier in Mail.app, you can select multiple mailboxes at once. This feature extends Smart Mailboxes, too, and that turns out to be quite useful in some circumstances.

For example, I have one Smart Mailbox set up that shows all messages from "John," and another from "Mary", two work colleagues. By selecting both of these mailboxes, I can see all messages from John or Mary, which is often useful. Another possibility would be All Unread Messages with All Today's Messages -- the possibilities are limited only by your imagination, and the scope of your Smart Mailboxes, of course!

[robg adds: This is a fast way to build a one-time "or" message list; if you want to build "and" combinations, you'll need to create a new Smart Mailbox set to match "all" following conditions. You can then set the first pop-up field to 'Message is in Mailbox' or 'Message is not in Mailbox' and specify other existing Smart Mailboxes using the second pop-up field. This hint contains an additional tip for creating more powerful Smart Mailboxes in 10.5.]