Mario is apparently the best assassin to have ever lived in Dragaera, allegedly called Mario Greymist for the number of people he has sent to Greymist Falls, the Dragaeran gateway to the Paths of the Dead. (Note: grey is also the color of death in Celtic mythology, as far as I know.) Of course, he does use a grey mist to kill the Emperor in Five Hundred Years After. He has not openly appeared in the Taltos novels, but does appear in Five Hundred Years After. He was mentioned in Jhereg. Vlad asked the Demon why Mario hadn't been hired for the job (of catching the guy who'd taken the Jhereg treasury) and the Demon said that Mario wouldn't work under a time limit. This implies that Mario is still alive, which isn't too surprising (he was young in Five Hundred Years After. (This also implies that Vlad may be considered the second-best living assassin, although Mellar possibly having gone East was a factor in hiring Vlad.)
There has been speculation as to whether we may have seen Mario in the Taltos books. The leading candidate was Kragar, not so much because there is any evidence that Kragar is Mario than because it seems Kragar could easily be the deadliest assassin if he did that sort of thing. Just that minor detail of virtual invisibility, you know? I mean, Cawti sat in his lap before noticing him, and he can be teleported by other people by accident. Certainly seems like he could be Mario.
On the other hand, I have been told by two people that Brust has been asked, and nixed this possibility. C'est la mort. Also that Mario and the Demon are not the same person.
There's also a scene in Jhereg where Vlad meets Aliera and she's very relaxed, which had already been noted by Morrolan. Given Paarfi's claims of romantic a relationship between Aliera and Mario it has been speculated that we just missed seeing him. This is complete speculation, though.
Right now I think that the Mario-Aliera thing isn't speculation, but I can't support that. I can, however, again nix the Kragar-Mario idea, and post-Issola speculation that Vlad goes back in time to become Mario or something like that. Brust has said, repeatedly, that "no one else is anyone else". And if that totally confuses you, you probably haven't read the right book.
I think the first section was pre-Orca. Now I can add that Kiera has mentioned not sharing Vlad's superstitious dread of Mario (and she'd actually mentioned him in Yendi, saying Vlad was harder to help "than Mario is to sneak up on", implying she's tried the latter.
Vlad mentions Mario a lot in a rather paranoid way in Issola, although it's hard to tell which mentions are jokes.
Otherwise the state of our knowledge has not really increased in the last 4 years.
The state of our knowledge has increased. There's a mailing list at dragaera.info, and Brust is on it, and has just said:
It is true that, in Jar-head, there is a scene where Aliera appears looking rather happy. In that scene, we have just missed seeing Mario.
and
>Have we met Mario face-to-face in the Vlad books (even if
>disguised as someone else)?
No.
Or see the original. And the other one.