If Apple launches a high resolution screen with the current A5 performance will lag as the speculated resolution is four times that of the current iPad panel. If Apple moves to a quad core Cortex-A9 that would help, but would impose a battery life penalty on top of what probably will also come with the new display. Likewise they may have to consider more cores for the GPU or a change of architecture. We have heard _nothing_ about such a processor even in any early rev. Similarly, memory bandwidth will have to increase which may pose penalties as well, and they may need to look at some dedicated video memory on board which means more die space without a node bump.
Realistically, contrary to the at times sophomoric chatter about on how Apple will somehow lose a swath of potential customers and brand cachet if they fail to launch a new iPad in September, a move to a higher resolution screen that conforms to Apple's MO (of maintaining a consistent user experience) would come out next year when either a modified CA9/SGX5 is available (or at smaller litho at least) or Cortex-A15 shows up which is possible for 2012. Failing to include a new processor architecture would make this an interim product at best and necessitate yet another iPad in the first half of 2012 anyway. I find the suggestion that Apple is at some great risk of being outclassed by Samsung et al on the basis of display resolution to be completely bogus. Apple competes against those platforms primarily in the basis of its software ecosystem, UI, and product integration...features which its competitors have woefully failed to address to a degree average consumers find credible.
Thus, in all likelihood, if a high resolution display comes out from some other manufacturer, most people will still buy the iPad 2 or, if they want a higher resolution screen, will wait for Apple to produce an iPad that has it. I don't know if it's geek egocentrism or something else, but it seems like a lot of the people clamoring about an iPad 2 HD fail to appreciate that many if not most of the people buying iPads, iPhones, etc do not see other platforms as truly viable alternatives irrespective of flashy features. The iPad 2's hardware is good enough to keep the device from seeming obsolete, so the pressure to release a high resolution replacement is rather limited.