Still extremely sore now four days later, but I'm fairly certain it's just the ends of the muscles in both the forearm and biceps. Hard to tell at first because they were so sore so close to the joints. Still can't extend my left arm all the way, but my right arm's fine now.
Haven't been this sore in ages, though. Typically don't get very sore, so I was a little more concerned than I probably should have been.
Dreamliner, I do a lot of lifts to failure, particularly biceps. I like to do machine curls and start at a high weight and just go up once and let the weight completely pull my arms down until I can't keep them up. Then I drop to the next highest weight and do it again, all the way through the stack. Works like a charm.