I am also running into the RPC lockout problem.  My setup is two HD6990s on two machines behind NAT with a single public IP.  I'm able to start two copies of phoenix on one machine, but then no other machine behind my NAT can contact mining.bitcoin.cz either for RPC or just to browse the web site.  At first I thought it was the large number of TCP connections created as I had 70 or so connections from the one machine on which phoenix was running.  However using DiabloMiner I'm able to launch an instance on each machine and the number of active connections jumps up to 130 or so.  I'm running the following on Ubuntu 11.04:

• Python 2.7.1
• pyopencl 2011.1beta3
• boost 1.42
• twisted 10.2

Thanks for the great miner.  It increases performance on my 6990s from about 530 Mhash/sec to 670 Mhash/sec using the BFI_INT instruction!