The open source Nvidia drivers going forward are Nouveau and those are designed from the ground up to be all Gallium-enabled, so that required DRI2.
The open source ATI drivers I am not sure about the status off exactly, but they certainly been working on GEM compatibility.. I think they got a bit sidetracked with the TTM-to-GEM transition, but they've been working with the DRI2 development early on.
Then, of course, Intel is leading the pack with their improvements.
I a took a look at what was going on on OpenChrome mailing lists and I found this that they were working on a pretty-much new Mesa driver to work with DRI2 and the unified memory management scheme.
So it looks like all of them are more or less well on their way to transitioning to the new model.
So that probably cover 98-99% of users with video cards capable of some sort of real opengl performance that have support of open source drivers.