It looks like clipping is broken in 6.5. the clip.c example doesn't clip properly , however when linking against Mess 5.1,6.2,6.4 and NVIDIA propriatary drivers it is fine.
OK, I've fixed this in CVS. You can grab the fixed file here: http://webcvs.freedesktop.org/mesa/Mesa/src/mesa/tnl/t_vb_cliptmp.h?rev=1.22&view=log
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