$ sudo apt-get -t squeeze-backports install build-essential 
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 build-essential : Depends: libc6-dev but it is not going to be installed or
                            libc-dev
                   Depends: g++ (>= 4:4.4.3) but it is not going to be installed
E: Broken packages

$ sudo apt-get -t squeeze-backports install libc6-dev
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 libc6-dev : Depends: libc6 (= 2.11.3-2) but 2.13-27 is to be installed
             Depends: libc-dev-bin (= 2.11.3-2) but 2.13-27 is to be installed
E: Broken packages

$ apt-cache policy libc6
libc6:
  Installed: 2.13-27
  Candidate: 2.13-27
  Version table:
 *** 2.13-27 0
        500 ftp://ftp.de.debian.org/debian/ testing/main i386 Packages
        100 /var/lib/dpkg/status
     2.11.3-3 0
        500 http://ftp.ch.debian.org/debian/ squeeze-updates/main i386 Packages
     2.11.3-2 0
        900 http://ftp.ch.debian.org/debian/ squeeze/main i386 Packages