iPhoto Library Manager - http://www.fatcatsoftware.com/iplm/ - has a great web site with lots of answers - from their FAQ for version 4 (should also work on older version I believe)
Downgrading a library to an earlier version of iPhoto
If you have a library from a newer version of iPhoto that you simply want "downgrade" to an older version of iPhoto, the easiest way to do that is with the "Rebuild Library" command. You will want to be running iPhoto Library Manager on the machine that has the older version of iPhoto you want to convert the library to. This will create a new library and import the entire contents of the original library into the new one using the older version of iPhoto. As with any rebuild, there are some items that will not be included (most notably, hidden photos, books, calendars, and slideshows), but if you have none of these items or don't mind losing them in the downgrading process, then this is a decent option for converting a library to an older version.