(Credit: Joe Raedle/Getty Images)

On the eve of Obamacare's primetime debut, Health and Human Services Secretary Kathleen Sebelius today heralded the moment as a "new day in health care for millions of Americans."

The administration says more than 2.1 million people have selected a private insurance plan through the state or federal exchanges since October - roughly half through HealthCare.gov and half through the independent websites run by 14 states plus Washington, D.C. It could not say how many of those who signed up were previously uninsured.

Officials conceded that they also do not yet know how many of those people have or will pay their first premium for 2014, thereby sealing their enrollment, and they declined to provide any geographic or demographic breakdown of applicants, a critical data point in evaluating the financial viability of the new insurance system.

Health and Human Services says 3.9 million people have been deemed eligible by the states for Medicaid and CHIP between October and late December, but it's unclear how many of those are new enrollments thanks to Obamacare. The total figure includes both renewals and those who would have been eligible under prior law, officials said.

The White House estimates 3 million more young adults will have insurance coverage in 2014 because of a provision in the Affordable Care Act that allows them to remain on their parents' insurance plans.

"The new law is transformation for our entire health care system," Sebelius told reporters on a conference call.

Meanwhile, the Centers for Medicare and Medicaid says it's still scrambling to resolve thousands of cases of individuals who couldn't sign up for coverage by the Dec. 24 deadline because of either technical difficulties or an inability to find a suitable alternative to their canceled existing plans.

A special hotline established on Dec. 20 to assist an estimated 50,000 Americans whose plans were canceled and who still had not enrolled in an alternative has received more than 2400 calls, said CMS spokeswoman Julie Bataille. Those with canceled plans are eligible for an exemption from the individual mandate and enrollment in special stop-gap catastrophic coverage, the agency announced this month.

A separate phone line has had about 7,000 calls from people who were thwarted by technical glitches in trying to meet the first enrollment deadline, Bataille said. Analysts are working on a case by case basis to determine which callers may be eligible for special enrollment with back-dated coverage to take effect Jan. 1. "People found eligible will have coverage on Jan. 1, " she said.

The Obama administration is bracing for scrutiny as those who succeeded in enrolling prepare to use their coverage for the first time. Senior White House advis4r Phil Schiliro says officials have been in close contact with their health industry counterparts to respond to an expected uptick in consumer confusion.

"Usually there's not a spotlight on what they're doing; obviously over the next few days there will be," Schiliro said. "Problems that have never gotten attention before will get some attention now."

"What we're stressing for folks is that if they get to a provider, if there is some confusion, to call the insurer. If the insurer is not able to resolve it, they should call our toll-free number that we've set up," he said. "If the operators have a problem resolving it at the call center, they'll go to a special case worker and those case workers will try to resolve problems as quickly as possible."