This certainly is a thing that's happening. Yep.
In a small tidbit of news, it seems that Valve has taken the plunge and finally allowed games that have monthly subscriptions on Steam. Their new system, which the game Darkfall: Unholy Wars is the first to use, seems to be pretty much in-line from what you'd expect from a recurring, subscription based service.
It's all pretty basic stuff, when it comes down to it. Nothing too fancy or innovative here. Yet, something is still interesting about it, and that something is thinking about what kinds of games may come onto Steam that will use this feature in the future.
I doubt that you're going to be seeing any of Blizzard's games on Steam or anything, that's what Battle.Net is force after all, likewise with Star Wars: The Old Republic due to Origin (yippee hurrah...) but this offers games that might not have had the proper resources to run their own subscription services an outlet to do so.
As for the first game taking advantage of the subscription service, Darkfall: Unholy Wars looks to be a little bit different from the average MMO in some regards. There seems to be a lot more emphasis placed on PvP, and from the looks of it the game will mostly revolve around such player interactions. This means that if you'd rather just explore the world and be left alone by other people -- which is an option even in MMOs in a lot of cases -- that this may not be the game for you. There is some assurance that those that don't want a PvP experience will be catered too as well, but it doesn't ultimately seem to be to the same extent.
Other points of interest include the fact that there's no leveling system in the strictest sense. Rather, abilities get more powerful the more you use them, which is kind of neat. Things like no friendly fire, a supposedly low ceiling for actually having a useful character (thus avoiding grinding), and the usual "get fat loots" are all potential draws to those looking for something a little different.
I kind of doubt that I'll be playing it any time soon, but hey, it seems like a fair and even fun thing on the face of it.
I guess only time will tell if Darkfall will find success with Steam's subscription services backing them, or whether other games will start coming out hoping to take advantage of it as well. There's a little bit of incentive since Valve is a trusted company in the gaming world and Steam is known as a mostly reliable service, occasional glitchyness aside.
Part of me is curious as to whether this will also lead to any future services that might be more in line with Playstation Plus or Xbox Gold memberships, where exclusive things and deals are given to those that are willing to shell out the cash. I'm not honestly sure how I'd feel about that. I suppose it would depend on what's being offered and for what price. I would like to say that I have confidence that Valve wouldn't screw it up, but it's shaky ground at best.
They could always offer early access to Half Life 3 I suppose, but then why bother calling it a subscription service when everyone and their dog would buy it anyways. Food for thought.