Well, I can't tell you where I read that -- but I have read reader reviews concerning the use of the 7000 in a G5, and no problems were reported.
Sorry I can't give you a link, but I'm 100% sure it works with the G5! ATi's site doesn't mention G5 compatibility, but it doesn't mention G5 INcompatibility, either. The 7000's specs indicate that it would be 100% compatible with the G5 (meaning it needs a 33/66MHz PCI slot, which the G5 has, and the G5 requires 3.3v PCI cards, which conforms to the specs of the 7000).
There are no "hardware specific" drivers for the 7000 so whether you put it in a G3, G4 or G5 it still uses the same drivers that come with OS X, and I can confirm that it works with both 10.2 and 10.3.