The design for Master Users (Users that can change from one bill to to another) means that having only 1 unnamed (CURRENT) cart per user is not sufficient.
The answer to this is to change from the hard coded CURRENT to be the currently selected billto customer number.
I'm thinking where ever we have CURRENT or reading the application setting of DefaultActiveCartName and replace it with CurrentUser.CustomerID.
The likelyhood of someone saving a cart with the customer number they don't even really know is remote enough to not worry about.