To which Debian package does /etc/nsswitch.conf belong?

This file belongs to the base-files package. You can use the following apt-file command to see to which package a file belongs:

sudo apt-get install apt-file
apt-file update
apt-file search nsswitch.conf

Check debian README files for explanation about some files in base-files package

cat /usr/share/doc/base-files/README

Q. After upgrading my system recently, I noticed that some files from base-files do not match the ones which are installed on a fresh install of squeeze. Should I not be warned about that?

A. Those files are configuration files, so they are completely under the control of the system admin. The files installed by base-files are just defaults. Changes in the default files are not important enough to warn the user, as it is also policy that prompting should be reduced to a minimum. This is also the reason they are not handled via dpkg's conffile mechanism.

In either case, if you want to "upgrade" those files, just look at the postinst for base-files (i.e. /var/lib/dpkg/info/base-files.postinst) and you will see how they are created and where their master copies are:

install_from_default /usr/share/base-files/nsswitch.conf /etc/nsswitch.conf
install_from_default /usr/share/base-files/dot.profile   /root/.profile
install_from_default /usr/share/base-files/dot.bashrc    /root/.bashrc
install_from_default /usr/share/base-files/profile       /etc/profile
install_from_default /usr/share/base-files/motd          /etc/motd

So, if you want your system to be as similar as possible to a newly installed squeeze system, you might want to sync these files manually.

dpkg --search do not show /etc/nsswitch.conf because it is not registered as conffile so it is a bit of a special case. For example dpkg -S /etc/deluser.conf can be used to identify package that owns /etc/deluser.conf file.
With path omitted dpkg --search nsswitch.conf actually show results.

I also like dlocate utility which is very helpful to find which package owns a file. dlocate is a faster alternative to dpkg -S.

apt-file is most useful to search for files in non-installed packages.