we just have to wonder more, till ios 4 for ipad is released!
As far as i can understand all ios apps are designed to be optimized and to support various devices. apps optimized for retina display are that, they are only optimized. so if the app is installed on iphone 4 it has hi res graphics, if installed on the 3gs or 3g it uses the low res graphics!
for the ipad its a different matter, coz basically it emulates the iphone screen for iphone apps and as of 3.2.1 the emulator will be depicting only a 3gs screen res!
when ios 4 for ipad is released, probably there are emulator optimization to emulate retina display graphics and apps would work at full res on an ipad.