As others have already mentioned, Half-Life has always come with an online multiplayer component (known as Half-Life Deathmatch, I think). It's just that nobody thinks of it when they think of Half-Life because... well, it's terribly unpopular and no one really plays it. This is what was modded in to Counter Strike, which then became it's own gigantic beast and insanely successful.
So yeah, technically this is nothing new. If, however, this means that they will be devoting more time for a Half-Life multiplayer product for the next installment, and splitting it up with the single player, I will be kind of mad. I kind of don't mind Half-Life multiplayer remaining the redheaded stepchild that no one gives a crap about.
Portal 2 multiplayer was fun, but tbh I would of just as soon had the single player campaign be released on its own, for cheaper, and not cared one bit about not having a co-op mode. Especially considering that the co-op is completely undesirable to play through it more than one time, which leaves a lot of people shit out of luck if they want to play it with their friends, who are already done with it. Like me. I have not completely the co-op, but everyone on my Steam friends has, and they are reluctant as crap to play with me since, you know, doing the same puzzles over again when you know all the answers and the other person is brand new to it is totally obnoxious. It's also not fun for the person who is new to it, since the other person solves it very quickly for them and there is no "discovery" stage. I'll be damned if I play with random people on a quick match system.
What I would love to see, if they are indeed going the Portal 2 route with Half-Life, is more of a hop-in-hop-out co-op, like Borderlands or L4D, where if your buddy leaves, a bot can take over, or you can just continue playing by yourself. Of course, this was not possible in Portal 2, since the puzzles required a 2nd person, and a bot cannot solve puzzles in the same manner. But it is really annoying when your partner crashes out or whatever, and you just get booted to the menu screen.
A Half-Life co-op where you have the possibility of a buddy popping in to play with you, as say Alyx, and then continuing with a bot Alyx or with Gordon solo if your buddy leaves, would be great.
What I do not want is FORCED co-op, or a multiplayer-only component that takes away from the single-player by taking up half the dev team's time and resources.