Read this announcement carefully.  You'll need to decide if it's profitable for your business to have your competition's listings right next to yours on your "Watch Reminder" and "Non-Winning Bidder" emails.  You'll also get exposure on other buyer's emails if you decide to participate.  Here's the announcement:

To increase the exposure of current listings and improve the finding experience for buyers, we will soon display similar active listings from other sellers on closed item pages. As they do today, buyers will first see other items from the seller who listed the original closed item in the "See More Great Items From This Seller" section. In addition, buyers will be shown a list of similar items from other sellers. This should be especially helpful for buyers who get to one of these closed item pages via a search engine.

Once launched, sellers who have not already opted out of having other sellers' items displayed on Watch Reminder and Non-Winning Bidder emails will be defaulted into this program. This will ensure your items receive maximum exposure on other sellers' closed item pages and that buyers who land on closed listings see relevant active listings.

Sellers will also have the ability to opt out of this program entirely, which means your closed item pages will not display similar items from other sellers, nor will your active listings appear on other sellers' closed items. Sellers can also choose to only allow the display of other sellers' items to any of the following types of buyers:

-Buyers who bid on, but didn't win, the item.
-Buyers who did not actually bid on the item.
-Both of the above.

You will be able to control these settings from the "Promoting Similar Items on eBay Pages and Emails" section of your Preferences in My eBay.