I have an iPad (Gen 1) and iPhone 4. Both were connected to iCloud and to iTunes, although the login for icloud differs from the login for iTunes. The app download on the iPad was not functioning, the app was in constant "waiting" mode.
I fixed this by logging out of iCloud and iTunes (Store) on both devices, and powering off both devices. Then I powered up the iPad, the device that was failing to download, and logged into iTunes (Store) first. The download worked. Then I logged into iCloud and checked the download. Then I powered on the iPhone, logging into iTunes (Store), then iCloud , checking the downloads on each device, and everything worked.
Two other settings need to be checked. For all of these tests iCloud backup has been turned off on both devices. Plus, automatic downloads of apps, music and books in iTunes (Store) is also turned off.
If you only have one device, I suggest logging out of both iTunes (Store) and iCloud, powering off, powering on, then logging into iTunes (Store) first then iCloud.
Hope that works.