Starting an app when iPhone in your pocket

Unintentionally? Difficult, at best. If a phone is put into a pocket unlocked, you may manage to activate an app.

If you mean intentionally, possible, yes, especially if you put the app on the first home screen in a corner.

Without being able to see the screen, I don't think it would be very easy to start an app.

Not very easy for it to happen by mistake, since it will only respond to your fingers touching the screen to swipe it on even if you inadvertently hit the home button.

To run an app, you'd have to hit the home button (easy), swipe in the correct place (not that easy), enter the code if it's password protected (very hard), tap the correct place to start your app (hard).

On the other hand, resuming ipod playing is relatively easier as you only have to double tap the home button and then tap the correct place for the play button. Still, it's not easy.