(Tuesday, 30.Aug.2011, Taiyuan China time) Until a day or so ago, my reputation ranking was 774. Then, suddenly, with no activity on my part, it is 874, exactly 100 points higher. What happened?
This is by design. See this blog post where the feature is announced, and the official answer on meta.SO (in the "additionally" section). As both resources mention, only one of the two accounts you are associating needs to have $\geq200$ reputation.
Also, if you go to https://math.stackexchange.com/reputation to audit your reputation, I am pretty sure the association bonus appears in the line
rep from bonuses:
although I am not sure what other bonuses exist... perhaps any bounties you've earned are included in this row as well.
By the way, I performed a recalc on your account (you can do this too using the "Trigger Reputation Recalc" button at the bottom of https://math.stackexchange.com/reputation) and that bumped it to 885. I'm afraid there's no good way of figuring out where the net difference of 11 points came from; generally, your "displayed" reputation can fall out of line with your "real" reputation, and the reputation recalc is just a manual update of the "displayed" reputation.