Why do you want such a thing?
Your OS will have to provide much higher resolution icons, fonts, etc, in order for the rendering to be appealing on such a screen. If you have a 30" @ 300ppi you're talking a monitor that's 9000 diagonal pixels so that would be about 7847 by 4408 pixels or 34.5MP. Compare that to, say, a full HD monitor at 1920x1080 which is just over 2MP. The memory and GPU power and bandwidth to drive such a monitor would be very significant.
That's not to say that we shan't one day have such a thing, but what is the demand at the moment? At 200dpi, we start to lose (as humans) the ability to truly distiguish the additional resolution - see
http://www.luminous-landscape.com/tutorials/understanding-series/und_resolution.shtml and certainly with a 30" monitor at normal viewing distances, it's gone well before 300dpi.
Don't get me wrong - I think most of us here would be happy to have stunning, super high resolution monitors, but you really should understand the differences between the resolution of a printer and the resolution of an LCD monitor and the useful resolution based on the performance of the human eye, as well as an understanding of the scaling of the resources necessary when you start increasing something exponentially (density of a square area...)