Simon Carless at GameSetWatch has the most cynical commentary so far: he says that Activision's CEO getting put in charge means they wear the pants, so to speak, and that even so, Activision is actually marrying well in this relationship-- as you might imagine, World of Warcraft's revenues are "staggering." EA, says Carless, is probably worried, but not actually threatened, by the relationship-- they're still bigger than Activision, and if Warhammer Online does well, EA won't have anything to worry about in almost any of their markets.
So in the short term, the talking heads at the company are probably right-- nothing will change. Vivendi and Blizzard are just securing their future in game publishing with a company that's shown it knows how to publish great games and do it well. Blizzard will still be Blizzard, and Activision will still be Activision-- they'll just both be called Activision Blizzard.
Where things may change, however, is a few years down the road, when Activision's management has turned over a few times, and Blizzard is working on new properties, possibly even a console property. Right now, the two companies have almost nothing in common, and that's why a merger works so well for both of them. But if Blizzard decides (or, in the future, is asked) to work on a console title, or if Activision decides that an old Blizzard property needs a new title not made by Blizzard, then we could see problems. Blizzard is known for taking their time and releasing quality titles, and in profitable game publishing (like what Activision is doing), the chance to do both of those things is hard to find.
For now, everything's fine. World of Warcraft isn't going anywhere, and odds are that any titles Blizzard has in development won't change either (as if we would know if they did anyway). But in the future, the bond between these two companies may grow stronger (or weaker-- maybe Vivendi, who still holds a controlling stake in Activision, will decide it was a bad idea and separate the two divisions), and that's when we'll see the effects of these decisions come into play.