I saw that post and it wasn't so clear as to whether LIVE traffic doesn't work altogether, or whether it can process the American non-HD LIVE service.
They're probably different traffic servers. When LIVE traffic in Europe has an outage, only American PLUS traffic has the same outage. American LIVE traffic is on a different outage schedule, thus I assume they're different servers.
All the "official" roaming in Europe is on the Vodafone network. In the US, Tomtom is not roaming on Vodafone (Verizon) since it is different technology. They are instead roaming on AT&T. Tomtom might be paying a large data bill to support US roaming, so even if it does work now, there's a chance Tomtom could block it if too many users use the service and it starts costing Tomtom $$.
If LIVE traffic works roaming in the US, it may be a reason for me to import a 940 from Europe. It would have the best of all the TT features currently offerred anywhere (except screen size).