I recently bought a Kobo and it wanted to connect to my WiFi before it would do anything. I ended up returning it.
However, there is a trivial hack to get around this where you connect it to your computer via USB and manually edit a sqlite database to put in some blank entries on the registration. There are lots of posts on how to do this online. After you've done this, it works offline just fine.
I'm looking for a new ereader and I don't want to have to register/sign in with anything to use it.