tedster - 3:44 am on Nov 5, 2006 (gmt 0)

If those two addresses resolve to the exact same content, but the url in the address bar doesn't not change over to the same domain, then you probably are splitting your link pop and Page Rank -- and possibly having one address filtered out of search results as well. If the addresses for one domain redirect to the other domain with a 301 status, then backlink influence should eventually pass right through.

You mentioned that this is a Yahoo store, correct? It is always possible -- I don't know for certain -- that Google has that kind of common situation all sorted out for Yahoo stores, even without the redirect. Perhaps some other member owns a Yahoo store and will know more definitely. As a general rule, you definitely need the redirect in place to see backlink influence pass over to the other domain.