I was using iPhoto '08, and was surprised to find that there was no apparent way to select a bunch of photos and create a zip file somewhere containing the originals. It didn't look like anyone had written a plug-in to do this, so I did.
ExportToArchive is a plug-in for iPhoto (tested with iPhoto 7.x and 5.x) that works with the built-in iPhoto export subsystem. You select some photos from your library and choose Export... (from the File menu in iPhoto 7, and from the Share menu in previous versions). You can export the files in a zip file, or a tar archive compressed with either GZip or BZip2. That's all it does, but it seems to fill a niche that wasn't filled before. Installers for both versions, along with a more detailed description of how it works, can be found at the above link.
If you don't want it installed for all users, then download this file and install it manually in ~/Library » Application Support » iPhoto » Plugins.
This is free software, released under the GPL (download source [32KB]); I hope this is useful to someone.
[robg adds: I tested this and it works as described.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20070830074100495