In Chapter 5, in the Orc temple, after fighting the minecrawlers and the Templar (Gor Boba), I'm apparently supposed to shoot some targets in an obvious trap room.
The first target works fine, sets off the trap, and opens the second target, but shooting the second target seems to do nothing. I assume it should open the gate behind where Gor Boba was standing, but it doesn't open. It makes the sound of activating, but does nothing that I can see.
This is really frustrating, as it feels I'm kind-of nearing the end of the game, and I get stuck...

I'm not sure if you are where i think you are, but if you're talking about the trap room with blood on the floor, i think there are 3 switches.
I think there's a third switch in a dark corner of the room and you'll have to use a torch or a light spell to spot it.

I've quit and reloaded my save several times, walked all around the temple and come back to it, and shot probably 100+ bolts from different angles.
I'll have a look for a third switch though.
EDIT: That was pretty random. loaded it up this time, fired 3 bolts at the second switch, and it actually triggered. Guess it's broken unless you hit it just right...
Thanks for the help guys.

This is a known problem. While reloading or going back to an earlier save may get it working, it won't always help.
For future reference, the other solution is to use Marvin mode. Open the character screen (S by default), type "marvin" (without quotes), then hit S to close the character screen. "Marvin-Mode Enabled" should appear in the top-left corner of the screen. Head on over to the similar room with two locked gates--the one the arrow switch is meant to open--and walk up against one of the gates. Press K to jump through the gate, then rotate the switches inside to see another mini-cutscene and open the gate into the next area. Go back up against one of the gates and press K again to jump back through. You now no longer need Marvin mode, so hit S, type "42", and hit S again (you should see "What was the question?" in the top-left corner of the screen). You can now continue on normally.