I rebuilt a new CentOS box from scratch and vboxguest hangs for about 3 minutes before continuing the boot - without the progress dots this time.
I obviously am missing something fundamental here since you are able to install this with no difficulty. What I changed this time was I added rpmforge as a yum repo (since dkms doesn't appear to be a part of the standard repos). Previously, I downloaded the dkms rpm manually.
# rpm -Uhv http://apt.sw.be/redhat/el5/en/i386/rpm ... f.i386.rpm
# yum install dkms binutils gcc make patch libgomp glibc-headers glibc-devel kernel-headers kernel-devel
And then ran the build.
# cd /media/VirtualBoxGA
# ./VBoxLinuxAdditions.run --nox11
Screenshot attached of the boot.
Shared folders appear to still work as I can see the host folder I shared. dkms looks like it has an effect here. What did you run to prep the box for the GA build? Maybe that's what is screwing up for me - I'm using repos I shouldn't be...