That's what happens when you connect a dual-link DVI monitor with a non-dual-link DVI source. It is limited to 1280x800, even though normal DVI can reach 1920x1200. It's because dual-link DVI pumps out half of the lines on one DVI channel, and half on the other. The 30" monitor's native resolution is 2560x1600, which if you'll notice is exactly twice the number of vertical and twice the number of horizontal lines you're currently getting. Since your mini only has one DVI channel, you're stuck at 1280x800. To be able to take advantage of that monitor's high resolution you have to get a Mac Pro or MacBook Pro.