It's likely that you've already caught this one, but just in case you didn't here's a quick rundown:

During the debate - make that 'comment wars' - things tended to dissolve into Opera vs. Firefox rather than actually discussing whether Opera 9 passes Acid2. For what it's worth, my opinion based on the Acid2 documentation is that Opera 9 does pass the test. The only remaining doubt is whether one or two lines should be left when you scroll the test. If the Acid2 page is scrolled, the scalp will stay fixed in place, becoming unstuck from the rest of the face, which will scroll. The documentation only discusses one line - that's an omission, not a failure condition.

Frankly, I think the intention of the test was to see if the smiling face would render in the first place. It does for me on all systems I've been able to test. My personal suspicion is that most of the failures are resulting from buggy installations (eg. final release installed over the top of a beta) or the page is zoomed or minimum text size is overriding the test - whether the user realises or not (it's easy to forget). Or possibly some other system glitch - there are lots of variables. If Opera 9 itself failed the test, I would have expected a more consistent result across the user base.