I noticed lots of high school kids wearing shorts to school when it warmed up to 27 degrees. And that explains why I often wear sleeveless clothes. Nearly all year. And Michelle Obama is my sleeveless hero.
There's clearly a need to balance professional with practical; but it really is miserable to feel over-heated at business or formal events when you don't have to be. And I know it's sometimes respectful to an audience/group to wear a suit so sometimes I have to appear to wear that much for a whole presentation/meeting - but then turn into an association version of Cher by un-layering before it's over.
I don't think Michelle's sleeveless style choice is just about her biceps. But, whatever the reason - hooray. It's now apparently fashionable for women to bare arms.