The idea of a "sandbox" RPG conjures a rather interesting image in the mind of the layman. The prospective player exists in an unformed expanse that is waiting to be formed by his actions. Their character sheet and the rules of the game are the bucket and shovel that the player uses to sculpt the setting into something truly awesome.
I actually tried to do something like this back in my Oz playtest game. Once the characters had made it to the Emerald City, I basically said, "Okay guys, you've made it to the Emerald City. What are you going to do now?"
Which failed. After a session or two of the characters not knowing what to do with all that sand, the party broke their oath to stay away from Utensia.
It was my recent acquisition of the Smallville RPG that helped me most with coming up with a good metaphor for this sort of open-world gaming. Character creation in Smallville is equal parts defining your character's abilities and defining their relationships, including drawing a "relationship map" that can become very complex as the process continues.
Items on this relationship map not only have lines linking them to the player's characters, but a rating describing how strongly each of the characters is connected to that minor character or location. Which basically explains why Clark Kent is always rescuing Lana Lang rather than Random Victim #4; His connection to her is stronger, so he is stronger in scenes and activities that connect to her.
So we have a complex map and a strong motivation to stay within it's framework. Sounds like pinball to me.
Rather than talking about sandbox gaming, we should instead talk about pinball gaming. Narrators (and increasingly, players) create a setting full of pins and bumpers for the players to interact with and bounce off of during play.