Up until recently best practices dictated that most web-sites would be designed to be easily viewable at a minimum resolution of 800x600px. But that appears to be changing...
With 85% of the web-going population now using a screen resolution of 1024x768px or higher a lot of designers and developers are starting to use that as the new de-facto standard.
The first commercial site I saw using the wider width was the U.S. Apple site and currently the other international Apple sites are still using the narrower 780px width. You will notice that this site is designed to take advantage of the new wider width at 990px wide.
While maintaining usability for as many people as possible is a very important consideration it's just as important to push the boundaries a bit in order to push people to adopt better technologies.
If you are a web-designer then consider the new width when designing your next site. If you need 100% accessibility or you think a high percentage of your users will be accessing from mobile devices then stick with the narrower width but otherwise go for it!