From the Twitter Blog:
Included in [the] list of [new locations] are a number of countries that will be getting Trends for the first time: Belgium, Greece, Kenya, Norway, Poland, Portugal, and Ukraine, plus more than 130 new cities in countries that already have Trends.
Prior to today’s announcement, trends were already available for more than 200 locations.
Learn More about Trends (Including How/Where to Change Locations and How Trends are Determined)
See Also: Trendsmap (3rd Party Service)
Real-time Twitter Trends info. This Trendsmap page lists all the locations where trends info is available.