I wrote a plugin for WordPress 2.x called Just Unzip. Just Unzip takes a zip file you upload from the “Write” page, unzips it, stores the zipped files in your upload folder, and associates the unzipped files with the current post. Just Unzip makes use of the PclZip php library (LGPL Licensed).
It is available for download at wp-plugins.org.
I wrote it because uploading several files to a single post is time consuming, and I happened to have zip files of the images I wanted to upload.