[carld]

As far as I know that's still correct. Once you turn the wifi on the book will expire.

Edit: or Ervserver may be correct. I'm not sure. It used to be, back when we borrowed mobi files from the library and ran Kindlefix on them to make them Kindle compatible, you could read them past the expiration date as long as your wifi was off. I haven't tested it since the new lending system started.