Wheezy is now stable and while that's all fine and dandy, when trying to install tor I kept on getting errors along the lines of
Reading package lists... Done Building dependency tree Reading state information... Done Package tor is not available, but is referred to by another package. This may mean that the package is missing, has been obsoleted, or is only available from another source
aptitude show torgave
No current or candidate version found for tor Package: tor State: not installed Version: 0.2.3.25-1 Priority: optional
apt-cache policy tor
tor: Installed: (none) Candidate: (none) Version table: 0.2.3.25-1 0 -10 http://ftp.iinet.net.au/debian/debian/ wheezy/main amd64 Packages
I got similar errors for e.g. wine and virtualbox.
The solution is in the output of apt-cache -- I set up apt-pinning a long time ago (for mpich2?) and forgot about it.
cat /etc/apt/preferencesPackage: * Pin: release a=testing Pin-Priority: 990 Package: * Pin: release a=unstable Pin-Priority: -10 Package: * Pin: release a=stable Pin-Priority: -10
Well, wheezy is now stable (and I am tracking wheezy only in my sources.list now) so the problem was quickly solved by simply deleting /etc/apt/preferences.