Sharing Sessions across domains

I want to share a session id used to identify users shopping carts
across multiple domains…www.domain1.com, www.domain2.com
wwww.domain3.com.

I am going to have all the data about the order stored in a database,
and pull the information with a session id.

All three sites will be sharing the same DB, but I need a way to share
the session ID across all the domains. Anyone have any suggestions?