Flagship Studios' Hellgate: London will share many similarities with the typical MMO, including a subscription fee. Make no mistake, Hellgate's online component won't be a string of PvP deathmatch servers, no, included in the multiplayer experience will be an expansive social system that includes guilds and raid-type gameplay.
Online, Hellgate will feature all of the single-player missions, plus additional modes, primarily focusing on group PvE instances -- limited PvP gameplay will be available at launch. Additional content will be continually provided by a dedicated team of developers. Sure sounds like an MMO, no?
Update: Co-publisher EA has stated that the online pricing model has not been officially decided (more here).