That looks like a problem. You have unstable version of gcc-4.4-base
(4.4.3-9) installed, but now your unstable/sid is commented out. So
when you try to install g++ which has dependency on gcc-4.4-base, it
is trying to revert the gcc-4.4-base to that of squeeze (4.4.2-9).
Install the package "apt-show-versions", that could show your what is
installed and from which version.
So either revert gcc-4.4-base to that of squeeze or uncomment the sid
line, do an update and install "aptitude -t unstable install g++" to
get sid version.