I have a feeling that we may very well see a 48xx card made available - there is profit in it for ATI. After all, the 3870 was made available and it is by no means a BTO option. And given that the GPU selection is the most universal complaint about Mac Pros - or will be again after the next upgrade - I can definitely see them taking advantage of the situation. Though admittedly, the whole situation with DisplayPort is a bit awkward.
Who knows, maybe we'll see cards with DP ports and DVI ports.
If I had to name my biggest problems with OS X, they would be: weaker GPU offerings and lack of options, lack of Nvidia optimization for same, and the lack of support for multichannel audio. I have to wonder if we may see them addressed at some point, because a lot of other complaints are being addressed in Snow Leopard. And hey, given the recent news on Blu-ray licensing, maybe that hole will be filled in, too.