Snarky political blog Wonkette nails it (pun very much intended) regarding the First Lady pregnancy rumours swirling around the interwebs.
From the entry:
In case nobody noticed, Mrs. Obama is kind of a big girl. She is a million feet tall and not a wilting cadaverous career anorexic with premature onset osteoporosis. This does not mean she is pregnant. It is very sweet, but also a little gross, that our nation is so taken with the fact that we have a young, attractive couple in the White House that we cannot stop fantasizing about them having hot procreative sex together constantly. But Michelle Obama just turned 45 years old and, unlike Sarah Palin, is smart enough to avoid a high-risk pregnancy at her age.