I have an nvidia quadro, and I have the rpm fusion X11 nouveau driver installed.
No, you don't. The nouveau video driver is part of the standard Fedora install. It does not come from the RPMFusion repos.
My understanding was that the nouveau driver obviates the need to install a new nvidia driver whenever the kernel gets updated. However, when I boot kernel 3.1.6 i get an error trying to start the X server. When I boot 3.1.5, the X server starts fine.
Wrong again. When you install the kmod-nivida or akmod-nvidia video driver from the RPMFusion repo, the nouveau driver kernel module has to be prevented from loading at bootup. Doing the corrrect kmod-nvidia/akmod-nvidia install method insures that the nouveau.ko does not load, and interfere, with the nvidia.ko kernel module. When you use the "nvidia" driver, you do not use "nouveau" for anything.
If you have kmod-nvidia installed and working for kernel-3.1.5, then when you upgrade to kernel-3.1.6, you must also upgrade the kmod-nvidia package that matches that kernel. If you don't, then there is no "nvidia.ko" to be found for the new kernel and you don't get an Xserver (graphical mode). Sometimes the kmod-nvidia package for a new kernel is a day or two late to arrive when a new kernel is released. In those cases, you either need to not upgrade to the new kernel until you see that there is a matching kmod-nviida package from rpmfusion to go along with it, or, install the akmod-nviida package.
What the akmod-nvidia package does is compile a new kmod-nvidia and kernel module on-the-fly at boot up time so you're not stuck without a nvidia kernel module for the new kernel.
To check which is your current video driver, do:
lsmod | egrep "nouveau|nvidia|vesa"
Is the picture a little clearer now?