I've had a few people ask me what the new fonts are in the Wordpress admin user interface, to tell the truth I have only a very small idea but will give it a try.
New menu and the text used in the "Add New Post" heading / title looks to be:
Google's Open Sans:
Google Fonts Open Sans
new body text looks to be: arial or helvetica or another sans serif font.
There's also some Tahoma thrown in there for good measure, a very popular font a few years ago. It's used for certain tabs on the page.