As uncovered by AppAdvice, the Gutenberg online library is listed in the iBookstore, so iPad users will be able to easily access their 30,000+ books in ePub format.
Of course, even if they didn’t, anyone can access the books via the Internet, but the access in the iBookstore alongside their paid offerings is a pretty big deal. It’d be akin to Borders opening public libraries in their book stores. (Yeah, Borders is pretty cool about letting you read every book in the place, but their “borrowing” policy is another matter.)
One potential downside: Project Gutenberg books aren’t necessarily in the public domain outside the U.S. Just because a copyright on a book has expired in the U.S., that doesn’t mean it’s expired everywhere else. I would not be surprised if Apple chooses to avoid potential copyright violation problems and limit Gutenberg access to U.S. only.