I struggled with this for quite a few hours as well on my CentOS 5.x boxen, and here's my solution.
It's apparent you need to update 'gcc' but natively upgrading via RPM or just grabbing RPM's at random causing some serious config mgmt issues on your server. I don't have time to compile gcc/g++ via source right now either. After grazing out in the repo for a while, I found that there is, indeed, an 4.x release of gcc in the base repo.
Do this (or someone with 'root' to do it in case of OP who doesn't have access):
# yum install gcc44 gcc44-c++ -y
...CentOS/RHEL have bundled a preview RPM of gcc-4.4.6.
Then when you go to do 'cmake' to build your release environment, do at least the following (your cmake params may vary):
# cd /path/to/OpenCV-2.4.3
# mkdir release && cd release
# env CC=/usr/bin/gcc44 CXX=/usr/bin/g++44 cmake -D CMAKE_BUILD_TYPE=RELEASE -D CMAKE_INSTALL_PREFIX=/place/to/install/ -D BUILD_PYTHON_SUPPORT=ON /path/to/OpenCV-2.4.3/
That will give you a successful build of OpenCV-2.4.3 natively with CenOS/RHEL 5.x.