TITiAN, the latest glibc version on RHEL/CentOS 5.3 is 2.5-34. RHEL/CentOS 5.x will not leave 2.5 branch (99% certanty). Not even third party repositories allow any newer versions.
RHEL/CentOS is considered locked down distribution for servers. It is based with stability and predictability in mind, not latest software. How ever, you can recompile some software from Fedora 6-10(maybe 11) but no mayor parts of the systems is wise to mess with, especialy since RHEL/CentOS 5.x will be supported until ~2014. Then rather use Fedora. It has 6-months release cycle but short life and support span.
wolverine.1408, if you need newer glibc (like me for wine 1.1.29), better install virtual Fedora 11 (you can use
VirtualBox for example) and enjoy safe main system with top notch guest system.