Let's say the estimates are correct and they have a few more than 25,000 subscribers at $6 a month.
I think 25,000 has to be a rather low estimate of the number of DDI subscribers, since there are currently 26,101 DDI subscribers who are
also members of the DDI group in the Wizards Community part of their web site. Anyone who has a DDI subscription but who hasn't joined the community won't show up in that total, so presumably the actual total of paying DDI customers is quite a bit higher.
Here's how that number (DDI subscribers who are also Community members) has changed over the last six months, with these numbers being noted down on roughly the 6th of each month:
September 2009: 7972
October 2009: 14392 [+6420]
November 2009: 18911 [+4519]
December 2009: 21570 [+2659]
January 2010: 23816 [+2246]
February 2010: 26101 [+2285]
Numbers in square brackets are the growth each month. It seems as if the growth rate has plateaued since December, but is now holding steady at a bit more than 2000 new DDI sub/community members each month, which seems pretty healthy to me.
While we're on this topic, I wish there was an easy way to buy DDI subscriptions as gifts for other people. I'd love to buy subscriptions for some of my players as birthday presents, but at the moment, doing that is rather messy. I'd like to be able to pay for a subscription, and provide an email address of the person I'm giving the subscription to, and for them to then get an email with instructions on how to activate it, set a password, etc. If anyone from WotC is listening, please add this to your list of ideas. You'll make at least a few more sales from me.