Actually it is a case of it is a bad idea to use it on ATI AGP cards until December. Nvidia AGP systems should work ok, though the cpu may be a little higher than a PCIE system (PCIE is simply a better bus design than AGP)
Once the new ATI drivers are released, AGP 8x machines should be ok with ATI cards. Based on testing AGP 4x is simply not up to the job which I guess is why AGP 8x and PCIE x16 were invented.
There may be a few PCIE systems from ATI which experience unusually high cpu usage but again the December drivers should resolve this as well. This high cpu usage will show up in particular with ObjectDock running, though some tooltips may also cause slowness.
If you do experience high cpu usage before then with ATI systems then ensure you have not forced antialiasing in the driver settings. Once the December drivers are out all should be fine with this as well.