ON SGS2: Only some apps won't allow installation, not all. However as soon as you unmount the SDCARD they install. Example of such an app is "TUBE MAP".
In addition all apps won't allow to be moved to USB storage. I am not using any app to move apps. I just go to settings>manage applications and select application and MOVE TO USB STORAGE, it will just attempt it and say "could not move app". None allows me to do so while I could always do so with GB MIUI, Cyanogenmod, Checkrom Roms.