PostGIS 1.5 Error with PostgreSQL 8.4
I solved it. I found the RPM's for GDAL, GEOS, and Proj4. They are located here: http://yum.postgresql.org/9.2/redhat/rhel-6-x86_64/

Since they are not packaged with PostgreSQL 8.4, you have to install them one by one. I hope this helps someone in future.