bookmarks at the back of books

At a given time I am reading at least two books, one light and one heavy (for my work) and ofcourse there are other books which I have read to few chapters and marked and left on shelf for later reading. Ofcourse, any time somebody moves my books, the book mark is dropped and moves to some page completely different.

Why not have a bookmark at the back cover of the book, on which I can write the page number I am currently reading and page number my kid is currently reading.

It could be made of e-paper (by phillips), which could be written and then erased as well. http://www.semiconductors.philips.com/news/content/ file_1054.html

It could be an RFID read-write tag on the back of the book, and the user buys a single RFID reader/writer (just once for all books), which he can use to program/read which page he is on.