indeed, the binary version of the client/launcher require glibc 2.7 (that is to stay upward compatible, 2.7 is the oldest version up-to-date glibc versions are compatible with)
to still play planeshift, you'll have to either update glibc manually (I'd really *not* recommend this), compile planeshift yourself (probably the easiest way) or update your distribution (may not be that much of a bad idea - ubuntu 6.x is pretty old)