I would guess that the major resistive-force acting against a large ultra-high PPI display is the number of pixels. Pushing all of those pixels on a mobile chipset may be beyond the capabilities of current chips, especially for 3D (a major facet of iOS software). As I understand it, desktop/laptop GPUs struggle to display at high resolutions.
The second resistive-force, I would imagine to be cost. 1024x768 displays at 10" are somewhat common, but something higher like 1920x1200 may be too expensive for the benefit of bragging rights or slightly clearer text.
The third resistive-force, I would imagine is the software resolution. As I understand it, iOS is resolution fixed (unless this is changed with the iPad SDK), and introducing a new resolution may break a lot of apps or split the young iPad market prematurely.
I would actually be surprised if the next iPad had a retina-type display. I would guess that there would be bump in processor spec, perhaps some cameras attached, the announcement of a complementary iPad-mini @ 7", and/or even dynamic LED backlighting for further power savings and AMOLED-esque contrast. But I would actually be shocked if the iPad-line had anything but a 1024x768 display for the next generation.