Thomas Nigro, a Microsoft Student Partner lead and developer of the modern version of VLC mentioned on Twitter recently that he heard Microsoft was building a brand-new browser. The IE team also hinted during a Reddit AMA earlier this year that the team had contemplated changing the name of IE.
Windows 10 will ship with both Spartan and Internet Explorer 11 but the IE will be there just for backward-compatibility's sake. Spartan will be available for both desktop and mobile (phone/tablet) versions of Windows 10.
Source: Mary Jo Foley