Hm, you're right, I hadn't noticed that RHEL 6 is supported with Plesk 9.5.4. I had heard that support for RHEL/CentOS 6 would not come to Plesk 9, but apparently it already has for RHEL.
But you're not forced to use Plesk 10, right? EL5 is still supported. And every 6 months? Come on, Plesk 9 was released in 2008.