The expanded traffic data is available for select cities (and in some cases main roads) in Austria, Belgium, Brazil, Canada, Denmark, Finland, France, Germany, Greece, India, Indonesia, Ireland, Italy, Luxembourg, Mexico, Netherlands, Norway, Poland, Portugal, Russia, South Africa, Spain, Sweden, Switzerland, Turkey, Ukraine, and the United Kingdom.
From the Windows Phone blog:
If you live in one of those places, pop open Maps and give it a try. It’s important to note that while the traffic menu option is now available in more places, traffic data might not be (especially if you’re driving through the countryside). So if your roads don’t change color when you tap Show Traffic, it means road info doesn’t exist for that area.
However, do note that the "show traffic" option will only be visible if your phone's region+language setting is set to one of the countries where traffic data is available. The amount of traffic data varies from country to country. For example, in the UK major (trunk) A roads are covered, together with most routes in major metropolitan areas, but in other countries coverage is limited to motorways and major routes through big cities.
This update is an example of how Microsoft is able to silently "activate" additional region specific features without a software update. This is achieved by turning "on" a feature in a central configuration file (something the phone automatically checks for, from time to time, when going on line), which "tells" the phone a given feature is now available in its region.