Mostly I was worried about the possible confusion that can happen when people visit a specific Instawallet linked somewhere, then later return to the site and don't notice that they are redirected to an "old" Instwallet instead of a "fresh" one.
How about creating a cookie only when a user visits the main site without a specific wallet? This should solve this problem. I think deprecating the cookies will be a significant decrease in convenience and cause many lost wallets.