I would be very surprised if they are unable to tie subscriptions to IP address, ranges or blocks.
I imagine it will be impossible to have multiple sessions open with the same user id, and there will probably be a limit to how many sessions can be opened with the same id within a given period of time (maybe 50 sessions in 10 minutes, something like that.)
Now, I can't see them actually tying DDI accounts to specific IPs, and disallowing the use of the account through a separate IP, but they should be able to track it, and make business decisions based on that data.
If accounts are strictly tied to a specific IP, I would hope WoTC sets up some kind of "Gamer Group" subscription rate, with a fixed number of user names and the ability to include a range of possible IPs, all at a lower rate that individual membership.