Hello
I don't have iPhoto in front of me, but search Help for Flip. I'd be surprised if it wasn't there. Select multiple images, and then Flip... if possible.
However, as someone else has suggested, look at Automator. I have 10.5.8. I've confirmed that Automator has a "Flip" action, under Photos. It seems to use Preview (not iPhoto), so there is a bit of complication to getting this right. (If iPhoto doesn't have a Flip function.)
1) Do a Backup. Just in case.
2) Export (using iPhoto) all of the affected photos to a folder (or a set of folders if you have organized them that way.)
3) Run your "Flip" Automator workflow.
4) Once you have confirmed all is at is should be - use iPhoto to import these images back into iPhoto into a new set of Events/Albums/Folders (however you want to organize them).
5) Once you have confirmed that these images are fine.... use iPhoto to delete the originals images.
Don't move/manipulate images in the iPhoto library with non-iPhoto apps - you risk corrupting the iPhoto database. The 'export' moves the images outside of the library, and then you 'import' them back into the library.
Good Luck...