Must say it is a good idea to have a "notify when back in stock" option.
My biggest problem with my site is when products get discontinued. I sell PC components and technology keep moving forward and as a result some items not only go out of stock but manufacturers stop manufacturing them so I have to delete those items. So I get the "page not found" as a result on those items. I know I should not delete products but what choice do I have if they will never be back in stock again?