So I've received a few emails since my post (I also sent out a few emails last night when I first started worrying about the issue) and here's what I've got so far:

1) In the latest build of Tiger released to developers (8A351) the Mac mini does not offer support for Core Image or Quartz Extreme 2D.

2) ATI has written me and explicitly stated that the Radeon 9200 was not designed with support for Core Image in mind.

Visually OS X Tiger should look identical on the mini as it would on any platform with Core Image support, the difference is that Tiger will continue to task the CPU for the rendering of all GUI elements instead of offloading it to the GPU. It appears that Core Image does require full fp support and it doesn't look like there are any plans to extend support down to DX8 class cards.

From what I have gathered, performance of the Mac mini under current beta Tiger builds is quite similar to performance under Panther, which is good to know, especially considering the beta status of Tiger. I did receive some reports saying that Exposé was choppier but that could be a result of a number of things. Dashboard performance is supposedly pretty solid as well.

More info as I get it...