From the beginning of development on my game I was hoping to release it as a universal app, but the gameplay simply doesn't work as well on the iPad. Also, it was designed to work on the iPhone screen, and the (even slight) difference in width to height ratio gives iPhone users an advantage over iPad users by seeing more of the path ahead. Not to mention it doesn't look quite right on the larger screen.
Hypothetically, if my game becomes a top seller, would it be a bad idea to have it only an iPhone app? Would it make it far less likely for my app to become viral? My app would still work on the iPad like any other iPhone app, and I plan on eventually releasing an update that supports both iPad and iPhone.