In fact, most, like Blizzard, actually tone down what they report because they don't list new accounts that aren't older than a month or so, internet cafe subscribers, trial accounts, cancelled accounts that still have time left on them, pay per hour accounts, etc.
I can't believe no one's given the correct answer to the OP yet...
" IT'S OVER 9000 ! "
On a serious note though, I have always like how EvE Onine gives the number of players logged into the server on the login page. Makes you feel like there are others around that that things are popping.
I have always found that game companies that keep their subscription numbers secret, leave themselves open to speculation and rumour. Once the game gets settled in, if there are 600k subscriptions, let people know. If there are 60k subscriptions, let people know anyways, otherwise people will make up their own numbers and those fake numbers often hurt a game more than they help.
As well, the number of people on the server would be cool at login too.