A long overdue thread if you ask me. Yes, it finally seems to work now.
No, it does not work out of the box with an HD5xxx - this is FAH remember

Steps needed to get the 6.23 Systray client to work:

1. After install & startup, you will notice it says "your GPU is not supported" even though the website claims "ATI 5xxx" as supported - to fix this, right-click the FAH icon in the system tray (while the error message is displayed), and hit "configure".
In the "advanced" tab, add "-forcegpu ati_r700" (without the "") to the command line on the bottom.
2. Now that the client is kind enough to start up, we need to adjust some environmental tables in order to stop the client from taking up a whole CPU core
There is a great explanation on how to do that here. Adding those variables dropped CPU usage for my HD5870 from 8% (1 HT core of a 12-thread hexa) to 3-4% which is acceptable, I guess.

Right now I am using the following values:

BROOK_YIELD: 2
CAL_NO_FLUSH: 1
CAL_PRE_FLUSH: 1
FLUSH_INTERVAL: 256

No idea whether they are correct, fast, or even stable, we'll see I guess

Report your results & findings with recent ATI cards here, and also tell me if you would do anything different.