iPhoto is better than that...
Mark Pilgrim: "Import of existing pictures is a pain; pictures can be tagged with keywords, titles, and comments, but then there are no search features to use these[...]"
There is a toggle switch to the left of the keywords box: Assign/Search. When in Assign mode, you assign keywords to photos. When in search mode, you search for photos with those keywords. I don't think there's a way to search the comments field (yet), but it's one-dot-oh so... give it time. And it's supposed to be simple enough for grandma to use... let's not turn this into a UI mess.