Thanks for the quick tips, guys. The reason I figured it was a problem with the O-ring is that I'd opened and shut the thing about half a dozen times in the process of rinsing and cleaning and it never leaked a drop until after I'd soaked the rings in PBW. The rubber mallet trick seems to have worked for now, but I think I'll pick up a replacement ring kit next time I'm at the store, just to be sure. Luckily, I won't actually be needing to use the keg for at least a couple of weeks, so it's not an emergency situation.
Next question: How do I get the outside of this thing cleaned up? There's at least two or three labels stuck on it, and it just plain needs a good polishing.