Description of problem:
After yum groupinstall "Desktop" and "Desktop Platform", xorg-x11-server-Xorg was missing, so there was no usable X system.
Version-Release number of selected component (if applicable):
RHEL6 public Beta 1 x86_64
"General Purpose Desktop" did not install this either.
"X Window System" is the group that you want.
This is the same as RHEL 5, FYI - there's a gnome-desktop and base-x group there that are separate.