In a crucial milestone, HealthCare.gov topped 6 million sign-ups, Centers for Medicare & Medicaid Administrator Marilyn Tavenner wrote Thursday in a blog post. The site saw near-record traffic with 1.5 million visitors this week, and the 800 number received more than 430,000 phone calls. "With four days left for consumers to sign up for coverage, we are working hard to ensure that our systems can handle the unprecedented demand as people enroll before the March 31 deadline," she wrote. The Congressional Budget Office originally estimated 7 million sign-ups by the deadline, but revised its projection to 6 million due to problems with the site's rollout. Blog post