After readying this little tidbit this morning – http://knowsharepoint.com/problems-creating-list-workflows-in-sharepoint-designer-2010-for-office-365-sites I decide to go out and test my own site.
Sure enough, Office 365 now appears to be hosted on SharePoint 2013, albeit with 2010 Experience. Below is a screen shot of my Office 365 site opened in SPD2013. Hopefully the option to update to the 2013 experience is coming soon!
