Well, its kind of both really. Dust 514 is really just an extension (not necessarily an expansion) of the EVE universe. What is really sweet is that both the EVE space MMO and Dust 514 will be connected. Up until Dust 514, every planet/moon in the EVE universe was used for mining minerals. Now, alliances and corporations (clans) have the ability to pay mercenaries to try to take control of the planets in every region and to try to take away sovereignty or steal resource or whatever the objective is for the mercs. The PC EVE MMO players can pay for ships, vehicles, and just straight pay the merc players for doing all of this work on the planets. So even though one is PS3 and the other is PC, they will both have an impact on each other (I would assume in real time). If there is ever a company that I would put my entire faith in, it is CCP who is the company behind EVE and Dust 514. If they are able to pull this off, this will be the last game we'll ever need to buy for a very long time.
When I went to their official site, I'm seeing tweets from pcgamer that says dust 514 will come out for the PC a year after the PS3 release. If they can manage to make this work, this will be amazing. Hit me up on TS3 sometime and I'll talk to you about EVE if you've never played it. I played it hardcore for about 2 years (I think) in a very good merc corp for both major alliances so I understand the game pretty well. EVE is the most complex, political, game where you can trust no one except close friends as all of the alliances have spies everywhere. I know for a fact our corp leader had 20+ paid active accounts that he used to spy on other alliances so we can figure out their movements within the universe and slaughter them as they were passing through various systems. God, just talking about it again makes me want to reactivate lol. But the game is so incredibly dense and complex I could never fully explain it well in a forum thread so hit me up in ts3 if you wanna chat about it.
Official site:
http://www.dust514.com/