If too much of a back story is provided, then it can limit the character that you want to develop. Instead, you will be playing someone else's character. I agree that the prisoner theme is getting a bit trite, but what would be an alternative blank canvas?
1. Amnesia as you wake up at a healers house (Done in New Vegas and it worked, but the role was defined to an extent before you got there)
2. Grow Up during the opening sequence (Done in FO3, Fable, and a few others but it is about as trite as the prisoner)
3. ?????
Wake up naked in the wilderness.
Start traveling in X direction, after a few minutes trigger NPCs that are chasing you.
Herd player to abandoned shack.
Player looks into mirror/wash basin.
- Insert character creation.
NPCs break into shack.
Fight ensues.
Defeat NPCs.
Go from there.