Downloaded app on iPad, now I cannot download it on iPhone I downloaded cinemagram on my iPad2 running latest iOS5.1 - now I want to install it on my iPhone 4S running latest iOS5.1, the install is greyed out

I even deleted it on my iPad - that did not help.

UPDATE iCloud had instantaneously downloaded it onto my iPhone but on the other screen I never look at.
You might be having this error because the app is already installed. If you have automatic downloads turned "on" for the App Store on your iPhone, iCloud will push the app to your phone if you buy it on another device.