The default value for GLX_STEREO is GL_FALSE when used in a call to glXChooseFBConfig(). If you don't specify this value then the call returns a chosen config correctly. However, at least in the case of Pbuffer creation, if you explicitly pass it the default value then the call will return no matches. This is incorrect behaviour because explicitly stating the default value should act the same way as not stating it.

Here is the minmalist list of attributes that I use to trigger the bug when calling the glXChooseFBConfig().

int * iattributes ....

iattributes[0] = GLX_DRAWABLE_TYPE;
iattributes[1] = GLX_PBUFFER_BIT;
iattributes[2] = GLX_RENDER_TYPE;
iattributes[3] = GLX_RGBA_BIT;
iattributes[4] = GLX_DOUBLEBUFFER;
iattributes[5] = GL_FALSE;
iattributes[4] = GLX.GLX_STEREO;
iattributes[5] = GL_FALSE;
iattributes[6] = 0;

I'm not sure when this started happening, but I have both MESA 6.5.1 and MESA 7.0 on my development server. 

Other relevant OS information:

Fedora Core 6 with all latest updates to date of this bug report. 
Running under Xvfb with varying command line arguments (doesn't matter what I use here, they always fail).

MESA 6.5.1 from the FC6 distro. Mesa 7.0 compiled from source download on Sourceforge.

Using pure software rendering. No DRI enabled or even compiled into the code.