In reply to th1:
I'm a continent away, but still I'd say absolutely do not chip at the rock to remove a cam. You should leave it if you can't get it by ordinary means. Sometimes they loosen---for example if it gets cold there might be a slight contraction of the metal that is enough to free up the cam---and sometimes someone else can get it out. I've heard about a few cases in which lubricating the cam surfaces has seemed to make all the difference. No oil or WD 40 please, just spit will do.
Since you've proclaimed yourself something of a cam novice, I'll presume to give some more removal advice beyond rigging nuts and/or slings to pull on an inaccessible trigger bar. Often the secret to badly overcammed pieces is to manipulate the cams with a nut tool. The reason is that you can compress a cam a tiny bit more manually than you can with the trigger mechanism.
So you get in there with your nut tool, retract one of the cams as far as you can get it, which should be a little more than the trigger will do, and as you do this, try to pivot the cam so that the side you are manipulating comes a little closer to the edge of the crack. Switch cams (on the same side) and try again, and then switch sides and try the same process on the two cams on the other side. Of course, try to keep the trigger retracted for whatever help it will give. One way to do this is to put wired nuts over each end of the trigger bar, clip them to your harness, and weight them. (Sometimes this by itself is enough to extract the cam.)
Progress is going to be measured in millimeters, so patience and a willingness to keep at it will be required.
Post edited at 22:32