we'll have to wait and see, PC version hasn't been confirmed yet (it's just a matter of time) but do you think it will have online multiplayer? the console version of SA had some sort of multiplayer while the PC game had nothing so there's a possibility it will happen again, personally I doubt it but who knows
either way I don't think they are focusing on making a really complex multiplayer, most probably it will be something really basic like deathmatch with 16 players or so, maybe cooperative missions.. however MTA:DM offers a whole lot more, being able to create your own gamemodes and fun scripts opens up endless possibilities
just because GTA IV will have multiplayer you think PC players will stop playing MTA? not to mention we have to wait ~1 year for the PC release