This is a combination of all my instructions on how to setup redmine on Centos. Its still a work in progress so please note any problems in comments.
links – more info to follow;
http://wiki.ossafe.org/display/misc/Ruby+on+Rails+on+CentOS+5.x
http://www.acqal.com/blog/a/article/first-whack-getting-redmine-running-on-centos-5/
http://www.how-to-linux.com/2009/02/how-to-install-redmine-on-centos-52-update/
http://moze.koze.net/?p=171
http://www.philliprhodes.com/content/installing-redmine-ror-centos