"ePubChecker" is a free easy-to-use program that runs your EPUB file through "epubcheck". Just drag and drop your book file, and click "Check".
Faster than uploading to a web site and more private as you keep your book on your own computer. Runs on Mac OS X 10.4, 10.5, and 10.6.
See the screenshot:
This app is built by my company, but uses the "epubcheck" v1.0.5 checking engine from Google/Adobe as required by Apple when submitting an iBook.