Microsoft have demoed a Windows 8.1 Enterprise machine running build 9415 today at the annual TechEd conference. The build is similar to what we have seen recently, and was being used to demo new Windows Server features. We recently saw leaked screenshots of build 9415 which didn't include a Start Button, however in these shots a Start Button is present.
This is because the builds were compiled from different branches, one from the partners branch and the other from the winmain branch. winmain is the main compilation branch Microsoft uses that implements all working features, whereas the partner branch typically doesn't include all features.
This is the first time the public have been allowed to see an Enterprise SKU of Windows 8.1. The demo didn't reveal much due to the nature of how the build was being used. The build was being used to demo new functions available in the new Windows Server 2013 R2 update, which is arriving as a preview later this June alongside the Windows 8.1 Preview.
It's exciting to see Microsoft talk more openly about Windows 8.1 now they've announced its existence. We can expect to hear more about Windows 8.1 in the coming weeks, building up to BUILD 2013 starting June 26.