If I rent a movie on my iPad can I watch it on my iPhone via the iCloud?

You can't. For you to have this kind of flexibility, you must rent the movie in your computer and then MOVE it to the device you want. As far as I know, you have to to actually plug the device in the computer.

Note that Apple in quite misleading in this matter. In the iTunes Store (in iPad) there is a link at the bottom "Renting & Buying Movies" and there you see a section "iTunes in the Cloud". There is no mention of the need to download the rented movie in a computer and then moving it to other devices. Quite the opposite: it suggests that it would work pretty much like Netflix (which REALLY lets you watch movies in any device, including Apple TV, iPad, PC and as far as I know, even PS3). It's amazing that Apple hasn't yet solved this basic limitation.