Not a userland API?

Open Source 3D drivers in Linux do their acceleration using userspace drivers. 

The *_dri.so drivers are loaded by your Xserver-side of things and then the in-kernel DRM
stuff is what opens up a hole for those userspace drivers to interact with the kernel. The
Linux kernel is now slowly taking on additional duties to manage display modesetting and
memory management, which should lead to help moving X out of the root account and better
display performance.

Even with very expensive cards there still isn't going to be enough memory on board to manage
a very large display with many applications open on a 3D desktop. So your going to have to
have some way to deal with intelligent way to deal with moving memory in and out of a video
card.