Make sure you have back-ports repository configured if you want a more recent version of the nvidia drivers which i highly recommend. Also the steps below need to be followed exactly so proper dependencies are met.
This will install kernel headers from stable
Step 1: apt-get install linux-headers-`uname -r`
This will pull in a more recent version of the Nvidia Driver from backports
Step 2: apt-get install -t squeeze-backports nvidia-glx nvidia-xconfig nvidia-kernel-dkms nvidia-settings
Step 3: nvidia-xconfig
This should get everything running properly and should make sure if the kernel changes that the module gets built.
Update: I have noticed in some cases when a new kernel is installed that dkms does not build the required module in this case try issuing the following command.
This issue has been fixed by installing the meta package for the kernel headers as stated above in step1