CentOS ISO
From FusionPBX
There is a new CentOS ISO planned soon. However, in the meantime if you want to test FusionPBX simply you may want to try using the old one. The old one is from April/May 2010 and therefore the FusionPBX in it is old and the Freeswitch is old. That said, they do work.
You can obtain the old ISO from FusionPBX for ex-Trixbox users blog