I believe it's possible to deduce who upvoted or downvoted on a post using the new First Posts review tool.
For example, here's a review I did on DIY.SE (the site where I'm most active):
The review history page (note: seeing the full history appears to be a 10k privilege) shows that I reviewed it at 23:36:21 UTC:
The timeline for the question shows a downvote on the answer on September 13th:
Clicking through to the user's reputation tab shows the downvote occurred at 23:36:17 UTC:
So if the only action on the post for the day is a downvote, and there was a review committed just a few seconds after that downvote, it's not terribly hard to deduce who placed it.
This may be less of a problem on SO and the other big sites, but for the smaller sites where there are few people reviewing, I believe this could compromise voting anonymity.