iPhoto '11 doesn't have the most sophisticated rules available when making Smart Albums. For instance, you can't refer to other Smart Albums in the rules. But there is a trick that makes them much more flexible than they initially appear to be.
When you use the contains operator in a rule, you can separate keywords by a comma. This rule will return the photos that contain any of the keywords, not just the ones that contain all of the keywords. So if the rule says, "Keyword is Cats, Dogs", the album will contain all photos containing either the keyword Cats, the keyword Dogs, or both.
This simple trick really lets you do more interesting things with the Smart Albums. Now you can make a Smart Album that can have all the photos that contain the keyword Cats or Dogs and are movies by having the rules "Keyword is Cats, Dogs" and "Photo is Movie" and saying that all the rules must match. Using the comma trick is the only way to make an album that mixes ANDs and ORs in its rules.