$ more /etc/apt/preferences
Package: linux-source-*
Pin: version *
Pin-Priority: 500

Package: *
Pin: release o=Debian, a=unstable
Pin-Priority: 90


$ apt-show-versions -u
linux-source-2.6.14/unstable upgradeable from 2.6.14-2 to 2.6.14-3
libgcc1/unstable upgradeable from 1:4.0.2-3 to 1:4.0.2-4
cpp-4.0/unstable upgradeable from 4.0.2-3 to 4.0.2-4
gcc-4.0/unstable upgradeable from 4.0.2-3 to 4.0.2-4
gcc-4.0-base/unstable upgradeable from 4.0.2-3 to 4.0.2-4
libstdc++6/unstable upgradeable from 4.0.2-3 to 4.0.2-4
libpisock8/unstable upgradeable from 0.11.8-14 to 0.11.8-15


$ sudo aptitude upgrade
Reading package lists... Done
Building dependency tree... Done
Reading extended state information
Initializing package states... Done
Reading task descriptions... Done
No packages will be installed, upgraded, or removed.
0 packages upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
Need to get 0B of archives. After unpacking 0B will be used.
Reading package lists... Done
Building dependency tree... Done
Reading extended state information
Initializing package states... Done
Reading task descriptions... Done