As discussed in this thread, with v1.5.1 or older, if your store doesn't use sideboxes, the only way for one of your customers to sign up for product notifications is to a purchase a product and request the notification on the checkout_success page.
This plugin adds the notification linkage on the product_info pages. No SQL updates, no core-file overrides ... just three template-override files.
It's just been posted to the plugins area.
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