You have to place an order every month. DCBS has a "pull list" feature but it doesn't auto-order for you, it's mostly an ordering tool to help you make your order quickly and make sure you don't forget anything.
Auto-ordering would be a nightmare because so many titles have multiple covers, high-priced incentive covers, and other nonsense that if you just said, "Send me the new issue of Batman every month," you'd end up with three different copies and some expensive shit you didn't want to pay for.
So no, there is no "subscription" per se, but ordering monthly is no big deal, takes five minutes. And if you forget, they usually send you a reminder email. And if you still don't do it, just email them the next month and tell them what you forgot from the previous month, and 99% of the time, they can add it to your order (they have a large brick & mortar store too). Like I said, customer service is aces.