This bug was just reported under Gentoo bugzilla and I can confirm the same 
problem.

Tagged git 1.6.3 worked for me as well as official 1.6.1, but upgrading to new 
official 1.6.4 breaks DRI.

Here's the relevant info:

In Xorg.0.log I see the following EE/WW:

(EE) AIGLX error: Calling driver entry point failed(EE) AIGLX: reverting to 
soft
ware rendering
(WW) I810(0): xf86AllocateGARTMemory: allocation of 16 pages failed
(WW) I810(0): xf86AllocateGARTMemory: allocation of 8 pages failed
(WW) I810(0): xf86AllocateGARTMemory: allocation of 1600 pages failed
(WW) I810(0): xf86AllocateGARTMemory: allocation of 1600 pages failed
(WW) I810(0): xf86AllocateGARTMemory: allocation of 9057 pages failed

These were not there before.

Here's the glxgears warnings:

ERROR!  sizeof(I830DRIRec) does not match passed size from device driver
libGL warning: 3D driver returned no fbconfigs.
libGL error: InitDriver failed
libGL error: reverting to (slow) indirect rendering
X connection to :0.0 broken (explicit kill or server shutdown).

Which is what you would expect when DRI is not working.

I upgraded the driver, rebooted, etc. I even re-installed libdrm, x11-drm and 
mesa to no avail (haven't rebuilt xorg-server yet). Though I do not think that 
any of that was necessary.

I am using Gentoo Linux with the following relevant packages:

sys-kernel/gentoo-sources-2.6.16-r13 (DRM not compiled/enabled in kernel - 
it's in libdrm and x11-drm)
x11-libs/libdrm-2.0.2
x11-base/x11-drm-20060608
media-libs/mesa-6.5.1_pre20060716
x11-base/xorg-server-1.1.99.3_pre20060802

Does this release have some requirements not met above? I.e. needs newer Mesa, 
Xorg-server, Kernel AGP, etc. 

If so, this should be clearly indicated somewhere (I couldn't find anything). 

Any help is much appreciated.