Make a decision as it comes up:
While I think that if this was meant to be a straight up area of effect spell, it would have been described as having a 'burst 1' inside of the stat block. However, the extra mechanic of it having to hit something and explode is a different story. You know, I'm reminded of in 3E, the fireball spell is described as a tiny red pearl that extends from your finger and blossoms into a fireball upon impact. This means if something invisible is in the way of your target, or as the book says if you hit the sides of the arrow slit you're shooting through, the fireball explodes.
This is essentially how force orb works. If the person your shooting at avoids the red pearl, or the force orb, there's no detonation and it continues on its vector path. So, there's nothing cheesy about aiming the fireball for the ground underneath the kobolds, in order to avoid the miss chance right? Right. Given, force orb isn't described in that same light, but I believe this is due to the cutting down of extraneous information in stat blocks on a whole.
This leads me to a question to someone experienced with house rulings: a wizard is inside of a room with a door that leads into a corridor that extends directly outwards. Then the door is busted down and behind it is an entire hallway full of orcs in a straight path. If the wizard quickly fires a force orb in this straight path, and fails to hit the first orc due to his good reflex defense, what happens? In a cinematic approach, it's easy to imagine the first orc, or maybe even the first few dodging out the way and then hitting the 3rd or nth or whatever. But from a rules standpoint how should that work? Should it work at all? I would think that the orb would continue down the hallway until reaching its peak range with the attack on the first orc applying against all orcs in the hallway. For instance, if the wizard throws a particularly bad Force Orb on an attack of 9, all of the orcs are able to step aside. However, on a roll of 16 only the lieutenants are able to step aside and a minion is struck behind them. I thought about orcs behind the first getting a reflex defense bonus depending on far they were from the initial target the of the spell, but that raises the question of should the initial target receive a bonus if he is more than X squares away from the wizard. Reflex defense based on amount of time received to respond to danger? Does the wizard's attack decide the accuracy and speed of the force orb? Oh lordie lordie. But, if any such house rules exist for spells like these, specifically about targets in a line, should any ranged attack work the same way?
An example I'd find easier to tackle would be the room and the door but the hallway outside is huddled against the outside walls of the room instead of extending outward. So, once the door is bust down, the wizard can see clearly an orc with a wall directly behind him. The Wizard tosses a force orb but narrowly misses, so the force orb hits the wall and detonates, meaning another attack against the same orc's reflex as well as against the reflex of adjacent orcs. What do you guys think?
(Edit: couple of typos.)