Hi Tom,
I’ve read about removing Open Sans support, and the reasons behind it. But actually my admin feels a bit slower after the upgrade. Not sure if the font change is related to that, or other upgrade related stuff. But, as you said, it is not related to GP.
But the Times like serif font I have in my editor now does have some WP upgrade and theme related cause somehow. Themes like TwentyTwelve show a sans-serif font, but other themes like GP show a serif font now. So it’s not caused by GP in particular, but somehow since the WP upgrade some themes show their own fonts in the editor, and others (like GP) don’t. I don’t know why the serif font is used suddenly. It’s not my system font (I’m on Win 7).
Anyway, I have looked into the add_editor_style() function as you suggested, and managed to get open sans in the editor. I like it much better than the new default situation. To get Open Sans back in the admin I used a plugin.
I added add_editor_style('editor-style.css');
to functions.php and * {font-family:"open sans", sans-serif !important;}
to a new file editor-style.css in my theme root.