Backports are recompiled packages from testing (mostly) and unstable (in a few cases only, e.g. security updates) in a stable environment so that they will run without new libraries (whenever it is possible) on a Debian stable distribution.
Add this line :
deb http://backports.debian.org/debian-backports squeeze-backports main to your sources.list and run :
To install kernel 2.6.38 I did ( amd64) :
apt-get -t squeeze-backports install linux-image-2.6.38-bpo.2-amd64
I needed the headers as well, but I got the message : The following packages have unmet dependencies:
linux-headers-2.6.38-bpo.2-amd64: Depends: linux-kbuild-2.6.38
Solution : just download linux-kbuild-2.6.38 from http://packages.debian.org/wheezy/linux-kbuild-2.6.38 and install it with dpkg -i . Now you can install the kernel-headers as well from squeeze-backports.