So, a build of Windows 8.1 Update 1 leaked onto the Internet last night. Buried in this build are early bits of what's being called Internet Explorer Enterprise Mode, a feature designed to "improve the compatibility of websites identified by your company".
There's not a lot to play with yet, but I tinkered with it a bit.
It appears to work similarly to the Compatibility View feature first introduced in Internet Explorer 8. One difference is that Enterprise Mode can be configured to consult a list of Enterprise Mode enabled sites via a file on disk or somewhere on the network. This file could contain a list of intranet resources, for example, that need to be rendered in a vintage document mode like IE8.
(Update -- Turns out, I missed Mary Jo Foley's write up on this feature in late January. Oops.)
There were also hints suggesting this mode would have the ability to run in its own sandbox, similar to InPrivate Browsing -- perhaps, and I'm guessing/hoping here, to allow for limited execution of older plugins and awfulware such as Java.
You can turn on a limited version of this feature via a tweak to the registry where the F12 Developer Tools hangs out. Simply open the Registry Editor and navigate to the key HKLM\Software\Policies\Microsoft\Internet Explorer\Main. Add a DWORD value of EnableEnterpriseModeMenuOption with its data set to 1. Finally, mash the F12 key in a new IE window instance and you should see a enterprise new browser profile in the Emulation area.