According to an official tweet last night and subsequent blog post this morning, Crypton Future Media had received many inquiries from users unable to activate their copies of Hatsune Miku V3, which had just been released yesterday. Yamaha, who runs the activation servers, also reported a similar situation last night. Shortly after noon, Crypton CEO Hiroyuki Itoh talked more in detail about the situation and their morning meeting with Yamaha through a series of tweets [1 2 3 4], stating that Yamaha had confirmed a much higher than normal amount of hits to their activation server(s) and are looking into expanding the capacity of the system. Apparently, activation for all VOCALOID products are done through the same server system, and hence this should be affecting other VOCALOID products as well; Itoh hopes for Yamaha to make a general announcement regarding the issue. He also noted that Hatsune Miku V3 can be used for up to 14 days without activation, and thus it would be possible to activate her later after the online activation problems have been resolved. Finally, he apologized for the inconvenience and promised to keep people informed of any new developments.
UPDATE: Yamaha has posted an update and apology on their website, stating that the servers should now be fixed and that they are looking into addressing the original cause (too many activation requests).