no. glibc-source is not necessary.
@ChimmyChanga
If you can't dist-upgrade your ubuntu install, then make a chroot for your srcds.
Installing new Debian systems with debootstrap
You won't need all of the directions there, and there might be better instructions elsewhere. Mainly, you're going to need to aptitude install debootstrap, and then debootstrap either lenny or a newer version of ubuntu with glibc 2.4 or maybe even 2.8. Once you get a distro debootstrapped under a certain directory, chroot into it. It's gonna be like a very sparse install, so you'll need to aptitude install certain things to get srcds up and running (not sure what). But then just get srcds up and running in that chroot envirnoment. Think of it as poor-man's virtualization =).
There are also tools to help with chrooting, mounting proc, carrying over users, etc. schroot is a good one. You could set it up so all you do to start srcds on your legacy server is to run the typical command with an alias in front of it telling it to use your chrooted environment.