Electronic Gaming in the Digital Home II study (via Worlds in Motion) only the hardcore gamers were into the subscription model, whereas the casual gamer showed "significant interest" in the free-to-play, microtransaction model.
Another part of the study found that, of the non-MMO players polled, 14% would be interested to play if they could play for free, whereas only 2% of that group was interested in subscription-based MMOs. As we always caution, studies generally aren't worth more than the pixels they're displayed on (unless it's a nice monitor, then they're probably worth less). In a recent polling at Joystiq HQ, games that are "free to play" were also seven times more likely to grab our attention than games that weren't free.
Said Director of Broadband and Gaming Yuanzhe (Michael) Cai: "World of Warcraft, with over 10 million players, exceeded expectations for subscription-based MMORPGs, but it's unlikely any other publishers will achieve the same in the near term using a subscription model." Of course, that's kind of a no-brainer even for WoW's competitors.