I, too, would like an app with exposure time longer than 4 seconds, but someone just told me that it might cause damage to the sensor or something. Maybe it could be done in the homebrew community; I know at that point it's in "at your own risk" territory, but for the sake of getting some really amazing night shots I think it might be worth the risk, maybe 10 seconds max or something.
As for the "just stack the images together in Photoshop" option, that doesn't work with every low-light photography scenario.