I think a computer based system for buying Bitcoin is more reliable than an app for a cell-phone.
Why? Bitcoin should be able to span to all digital mediums... there are African communities that have all of their currency on their mobile phones.
I think this is a feasible idea - text your deposit address and amount to a number. They then charge your cell phone bill (just like those text-to-donate commercials). The biggest obstacle is phone carriers and people paying their phone bills.