The Nintendo 3DS has a feature called StreetPass where, even when in Sleep mode, the 3DS detects other 3DS nearby and swaps user tokens and simple information, including the last game you played. This is, of course, entirely optional.
But a similar scheme could work for eReaders to swap the book title and cover thumbnail amongst other eReaders in the vicinity. All in the background. Then the curious could get their fix, and those who want privacy can simply not enable it.