Actually, now I'm having this problem too! It must be contagious, from reading this thread!
With my NST, it happens if I am reading a book and then push the N button at the bottom to check my battery level (go to Settings). When I then click the book icon on the screen to return to the book I'm reading, it send me back to page 1. Grrrr!
I found a work-around for this particular condition and maybe it will work for others. If I go back to the Library and pick the book from there, it returns me to the last page I was reading.
I also would say this seems to only happen on a side-loaded book. Any book I have purchased from B&N seems to work OK. At least for now...