@DustinT :
My response above, prove that there is a difference of font smoothing between Windows & OSX and after some searches via google :
http://tonymacx86.blogspot.com/2011/03/improve-font-smoothing-in-mac-os-x.html
I found several threads about this in macrumor's forum.
I'm actually using the HDMI -> DVI convertor provided in the Mini package, could it be the reason of a lack of sharpness added to the fact that my Dell display is not 1920 HD like the newest product in the market.
Maybe higher density/resolution display could avoid that problem. when I tested the iMac 21,5 & 27", text was gorgeously crisp but I rather have a matte screen.
So for now, playing with the default smoothing, it seems to work fine. I'm still curious to see the display quality with the AMD CG on the high en mini.. OSX Lion could benefit from the CG & OpenGL for smoother transitions, less lagging in Photoshop or whatever, who knows ?