If you can't figure it out, just box up the ATI card and send it to me. I'll test it for you for the next 3 years or so.

What version of BOINC?
Does BOINC recognize the type of ATI card or is it one of the new 6xxx cards reported as "unknown"?
What GPU preferences are set in boinc manager?
Did you already override the don't crunch when xxx % of the CPU is used?
Current versions have separate run always, run by prefs, etc. for cpu, network, and GPU. If used to running older versions, it may have started the WU but suspended it as soon as it started.
What projects/apps are you running? Opt or standard?
If you check the slots folder for the ATI project, is there a stderr.txt or stdout.txt there? If so, any hints inside of them as to what it is doing?