Is it possible you're using a RHEL 6-Beta DVD on a RHEL 6.0 system?
It looks like RHEL 6 has always had glibc 2.12 but the beta release had glibc 2.11. I really can't find a definitive source that says what the 6-Beta had but find mentions of 2.11 on 6-Beta around the web like here and here. All of the CentOS src.rpms for 6.0 to 6.3 are glic 2.12 so the final release has always had 2.12.
Is it possible you initially installed from the 6-beta DVD but have upgraded to a newer RHEL release since then? If so, you really can't use the packages from an older RHEL DVD. If you're just trying to install gcc, you can run
yum install gcc to get GCC 4.4.x. In general, installing through yum is preferred over the DVD since yum will automatically fetch the latest RPMs whereas the DVD might have an older version that has some bugs. If you really want the DVD method, you'll need to get a DVD that matches the RHEL 6 release you have installed.
cat /etc/redhat-release will tell you what version of RHEL you're running. I'm guessing you're on 6.0 since the version of glibc currently installed is from November 2010 (you should look in to upgrading to 6.3 at some point). As for how to tell what version the DVD is, I'm guessing if you boot from it, it will say RHEL 6 Beta or something on the splash screen. Maybe read the docs on the DVD to see if it references being a beta?