I wouldn't have any issues with it happening as either of those two, but the ARB has done it the way I described in the past. All of the fixed function stuff was removed from OpenGL 3.1. There is an optional extension that includes all the old stuff. Which is supported by all hardware vendors that I know of, so in practice it meant nothing.
I find it hard to belive that the ARB would take the time to create an emulation layer. They can't even get their reference GLSL compiler straight. A lot of it I belive was written by Valve because they had a dire need for the functionality it does have. So who in the ARB would have a dire need to help write a compatibility layer? The most probable candidates would be the vendors themselves. However, already having the code in their existing driver, it would surprise me if they bother.
Nobody thinks it's sexy to fiddle with the old stuff, so my bet is that they'll chose the path of least resistance and implement compatibility the same way as they did in 3.1.