Shared Cookies on different domains...

I ran into this problem few days ago working on my custom store code for a client. They basically have a couple of online shops and want to make single user database and singe shopping cart for certain domains.

Problem was how to enable single shopping cart for users that are browsing domains anonymously. After couple of hours of brainstorming and trying few things out I think I found what works well for our scenario.

I create a simple user fingerprint based on their IP and browser. This is then linked with the shopping cart identity. After you open a second domain the identity will be assigned from the fingerprint. If it does not exist yet it is created.

simple solution to a complex problem that works well.
After they finish checkout process we create user name for them and after that shopping cart is shared based on username.

hope it helps some1 in similar cases...