Seems that there is some confusion about the active subscribers, registered users and copies sold.
Some news pages and blogs report, that Warhammer archived 1.2 million users so far. This is not 100% correct. Actually it's 1.2 million copies delivered to the shops/actually sold. But in reality "only" 800.000 copies have been activated, which is still a pretty good number and even outnumbers World of Warcraft during it's first 2 months. It's highly probable that Warhammer will break the 1 million by the end of this year, which indeed will make it a success, compared to recently MMOs such as Lord of the Rings Online,Vanguard and Age of Conan, which didn't passed the 500.000 mark.
However, 800.000 registered copies, doesn't mean that the game has 800.000 subscriptions. There are many people who were disappointed and didn't subscribe after the first free month included within the retail box. So far the real number of subscribers is unknown.
There are first indications that there is a pretty fairly number of people who stopped playing, which could be observed on some European Servers which are there since the pre-release start where preorder customers could start a few days before the actual release. This servers were pretty full and European servers still don't have a character transfers, so they are quite good for to observer such changes.
Some of the servers, like Carroburg and Erengrad, where full even weeks after the cloning process with queues of 200-400 resulting in waiting times of over 90 minutes. At end of October and beginning of November, the queues suddenly disappeared. No big surprise here, as that's around the time, where the free trial month is expired and many people didn't extended their subscription, which is a good indication that there are indeed many people who stopped playing Warhammer.
Also, on Erengrad it's pretty hard to join a Scenario, as the waiting times there are pretty high which indicates that there are not many level 31-40 characters there who sign up for scenarios. The situation on Carroburg on other side looks much better, the waiting times are around 2-5 minutes depending on the daytime.