With the latest 5.0.2 update to iPhoto, Apple has fixed a gripe I've had with the keyword filter partially documented in this hint.
Prior to 5.0.2, clicking multiple keyword buttons in the keyword pane would result in an OR filter. In other words, each additional keyword clicked would add pictures to the search that had any of the selected keywords, instead of all of the selected keywords. This is non-intuitive behavior (at least it seems that way to me).
In iPhoto 5.0.2, the OR filter has been replaced with an AND filter. This means that when selecting multiple keywords as a filter, the photos/movies that are displayed must have all of the selected keywords rather than any of them. The OR (or any) filter can still be accessed by shift-clicking on multiple items (their titles turn purple to indicate this).
These searches can also be combined with NOT filters by option-clicking on the buttons: option-click performs an AND NOT while option-shift-click performs an OR NOT.
[robg adds: The logic can get quite confusing (to me, anyway), but I believe the above description is right. At the risk of embarrassing myself, I believe it works like this for two keywords, A and B:
Mac OS X Hints
http://hints.macworld.com/article.php?story=2005041900454453