The change to instant rep was rolled out gradually to all users. Yours was applied on the 28th of March, however allegedly your rep only changed by one point (and it went up).
However, downvotes are now free for questions so you won't see any rep change because of that. I just did a test by throwing a -1 on a spam answer, and I saw my rep change immediately after refreshing the page.
Hopefully a dev will see this and cast more light.
Just noticed you did spend 500 rep in September 2011 on a bounty. Kudos to you for throwing out a big bounty...