I experienced exactly the same issues with both the nvidia proprietary driver not being installed correctly and nouveau crashing the xserver frequently (for me, it was every time I started chrome or firefox).
I am much obliged to both of you for the solution!
However, I am disappointed that Ubuntu let such a critical and hard-to-diagnose bug (for non-experts) get into the 12.10 release. For me, this started when installing the nvidia 3rd party driver as the first step after a clean install of 12.10.
I've been using Ubuntu since Dapper and have never had this issue before.
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