From today's People Magazine:
When a new First Family moves into the White House, staffers typically gather in the East Room for a chance to formally welcome the incoming lady of the house.
But this year's meet-and-greet featured an element of surprise: As a casually-dressed Michelle Obama arrived for the event Jan. 22, White House pastry chef Bill Yosses and assistant pastry chef Susie Morrison wheeled out a cake topped with fruit, candles and best wishes spelled out on a plaque
http://www.people.com/people/article/0,,20255483,00.html
Looks like she greeted the "help" in a t-shirt.