Ruby 1.8.7 is included in RHEL 6.
If you run an older version, then you could try to download the corresponding srpm and recompile.
From my workstation:
$ rpm -qi ruby
Name : ruby Relocations: (not relocatable)
Version : 184.108.40.2069 Vendor: Red Hat, Inc.
Release : 5.el6_0.1 Build Date: Wed 17 Nov 2010 08:14:57 PM CET
Install Date: Tue 01 Mar 2011 10:52:22 AM CET Build Host: x86-007.build.bos.redhat.com
Group : Development/Languages Source RPM: ruby-220.127.116.119-5.el6_0.1.src.rpm
Size : 1875128 License: Ruby or GPLv2
Signature : RSA/8, Fri 31 Dec 2010 06:34:12 AM CET, Key ID 199e2f91fd431d51
Packager : Red Hat, Inc. <http://bugzilla.redhat.com/bugzilla>
URL : http://www.ruby-lang.org/
Summary : An interpreter of object-oriented scripting language
Ruby is the interpreted scripting language for quick and easy
object-oriented programming. It has many features to process text
files and to do system management tasks (as in Perl). It is simple,
straight-forward, and extensible.
$ cat /etc/redhat-release
Red Hat Enterprise Linux Workstation release 6.0 (Santiago)
It looks that 1.8.7 made it into the final version, it could be absent in beta. If you have Red Hat subscription you should be able to get it. Otherwise your best bet is to wait for CentOs, I think.