Some of our latest android projects reflect the shooting time, and "SmartScope" is our first one. Its up now on Google Play. It basically turns a phone into a rifle or pistol scope.
The mechanical attachment is fairly easy, using a picatinny rail adapter. On the long rifle, though it is more tricky. The other problem when using with another 'traditional' scope, is parallax - the phone's camera has trouble seeing and focusing properly with the other scope's view. Not too tough to get around, though.
We've added almost 2 dozen reticles, including some for a pistol - both vertical (portrait) and horizontal (landscape). It was tough dealing with the phone's camera in portrait mode on older Android OS (2.3 and earlier).
It is alot of fun to use on a pistol. Works great with a Glock, which already has a small picatinny mount on it.
With the high refresh rate on smartphones (how quickly and often folks upgrade) there are already alot of very powerful, older smartphones out there. Many of our projects are oriented around utilizing the computing and sensor power on these phones.
One other cool way we've been using this app is for data collection - it will record the noise levels (sound) and recoil accelerations or "kick", while shooting. We'll post some of that data soon, too.