Base on the statistics by W3school, more than 90% of the internet users are using a display resolution with 1024×768 pixels or more. If you study the trend, more and more users are switching from lower resolution to higher resolution display over the years. In 2000, only 4% of the users were using a display of more than 1024px. The overall users in this category kept increasing and it reached 57% as of January 2009.
All my themes were designed base on 1024px and above. Some of the themes like Maple Leaf (preview) and Beach Holiday (preview) even show nice background when viewed with 1600px.