An formal announcement from Sebelius will come at some point on Friday, according to Bloomberg's Alex Wayne, Julianna Goldman, and Drew Armstrong. The New York Times' Michael Shear reports Obama accepted Sebelius resignation this week. There's no indication when she initially submitted her resignation papers.
The search for a replacement will be short lived. The Times reports Obama will nominate Sylvia Mathews Burwell, the director of the Office of Management and Budget, to replace Sebelius. Whether or not Burwell will ever be confirmed by the House is another story.
Sebelius will be remembered as the embattled member of the Obama team who oversaw the rocky, but ultimately successful, rollout of Obamacare. Through the many difficulties the program faced during its launch, the blame constantly and consistently fell on Sebelius. Sebelius ducked and dodged rumors and calls for her resignation over the last few months, but she stuck through the job until healthcare signups achieved its 7.1 million sign up goal on time.