Now the people at Google Blogoscoped are reporting that the usage guidelines for Google Book Search may impose copyright-like restrictions on works that aren't copyrighted! They state:
Google Book Search allows you to download full PDFs of books which are in the copyright-free zone of the public domain. I think Google, as well as the libraries which offered their books to be scanned by Google, deserve credit for this. However, Google wants to impose some restrictions for those books, and in their usage guidelines ask you to:
Make non-commercial use of the files ... we request that you use these files for personal, non-commercial purposes. ...
Maintain attribution ... Please do not remove [the Google “watermark"].
Upon further investigation, however, this seems like more of a problem with unclear language than an actual copyright issue. This post from the Blogoscoped article's comments section seems reasonable to me: What it says is that Google isn't trying to stop you from copying the content, which is in the public domain, but from copying and profiting from the scans they made.
There is actually a lot of good discussion going on in the comments section—definitely worth checking out. I have to admit, though, if Google Books seems confused, I'm beyond perplexed.