As reported earlier the Apple Tablet could be named iSlate. MacRumors and others have found information in trademark and domain registrations that point to Apple owning iSlate trademarks and domain names since 2007.
Seth Weintraub over at his Computer World blog raises the thought that iSlate could have been the backup name for the iPhone. In 2007, when Apple launched the iPhone it became known that Cisco actually owns the trademark for iPhone. Both companies came to an agreement, but it could also have turned awry. In that case Apple would have changed the name of the iPhone.

iSlate could have been registered by Apple to become the alternative name of the iPhone.
This could have been the original motivation of Apple to register iSlate, but I think it fits perfectly now for the Apple Tablet and Apple might have just realized the same thing.