IE7 already has a 25% US market share
Since Internet Explorer 7.0's release in October and its subsequent rollout via Windows Update, the browser has become the second-most used after IE 6.0 in the United States, according to Microsoft's IEBlog. The blog says more than 100 million users have installed Internet Explorer 7.0 so far, and that 25% of visits to U.S. websites are now made using the new browser. By contrast, Firefox's U.S. market share was slightly below 16% six months ago, although that number has likely increased since then.
The IEBlog folks reckon IE7's market share will keep increasing, too, especially with the retail launch of Windows Vista at the end of this month. IE7.0's global market share will also increase as Microsoft rolls out more localized versions of the browser. The IEBlog doesn't quote any worldwide market share numbers, though.