1.5.0, 1.5.1, 1.5.2, 1.5.3, 1.5.4, 1.5.5
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.
You can see it in action below (albeit customized!); it's the "Update Notification" link.