This is a performance thing which is only noticeable when you need more than 2 1080p movies at the same time.
Gt120 gtx 285 quadro 4000 will play close to 6 hidef movies on current mac pro
ati 5770 5870 only manage 2 and they are glitchy.
As this is a very unlikely requirement for normal users even those doing games
I would say it's not a high priority problem for apple to find locate or fix
If they even see it as a problem - and not the byproduct of a design decision
related to ati cards
I would not wait for a fix but buy a gt120 as interim solution- but the gt120 does not have great overall performance
You can plug a gt120 in at the same time as 5770/5870
But you can't use them both in catalyst