Seriously. If the iPad had a 326 PPI display in the exact same physical size, its total number of pixels (2529 x 1897 = 4,797,513) would be 17% higher than that of a 30" Apple Cinema Display (2560 x 1600 = 4,096,000) -- and a 30" ACD, in its current large form factor, still costs $1,800. Try cramming all of that resolution down into something that's not even 5% of the volume of the ACD and let me know what you think it would cost.
In other words: Give it time, people. I'm sure the 4th or 5th generation of the iPad will have an insanely high resolution comparable to the iPhone 4 in terms of PPI.