Well, assuming the NPC doesn't win any contested rolls to notice what's going on, they'd see and notice the weapon, and the attack, but not that it hit them. At least until the next round. It basically would give you a surprise round, and the NPC may be a bit confused as to what had cut them, possibly suspicious of magic, but would otherwise play out the encounter normally.

If the NPC was distracted, however, and you could manage to draw, mosquito bite, and hide the weapon again, well they'd have no idea wha was going on and it gets fun very quickly

Is there a feat or skill that would allow for a concealed attack as well?

For that matter, I imagine the game would rule against even seeing the attack, if you managed to strike from the back, right?

There's no facing, so you can't strike from the back (though I'd personally allow it if you were out of combat, and the target was distracted). If there's a crowd, you can use it for cover to hide. I might require an additional sleight of hand check to keep it from being obvious that you were hitting someone with your weapon, mind.

Normally, I'd say it would require a successful hide check, within striking range of your target. This usually requires a class ability such as camouflage or hide in plain sight, an invisibility spell, or something that provides concealment such as poor light, fog, or a crowd. There's also the matter of not being noticed afterwards, which carries something to the tune of a -20 on that hide check. Some feats that would help with this are Crowd Tactics, from races of destiny, which could be had at level 2 (+4 to hide in a crowd square) and Shadow Striker from the PHBII, which offers a number of special actions that seem to fit your concept.

That's an interesting rules question, isn't it? If they don't notice that an attack even hit them for a round afterwards, is there still a -20 penalty on the attack? You normally take the penalty on an attack regardless of whether it hits or misses, but if the idea behind a feat or ability is that the attack isn't even noticed right away it makes me think that it could be a very reasonable argument that there is no penalty to that hide check.

I'm going to have to go reread mosquito bite, see if it delays noticing the attack or the damage...

They don't notice the hit. It does say they think you swung and missed, mind. However, if I were GMing it, I'd probably let the player get away with it, assuming he was hidden before the attack and afterwards moved away.

Aye, a hidden attacker is treated the same, mechanically, as an invisible one. Whereas with an invisible(hidden) creature the target doesn't notice the attack, only the hit; and with mosquito bite they only notice the attack, not the hit, I believe it's reasonable to assume that when an invisible(hidden) attacker uses mosquito bite, the target would be aware of neither.

lol if you liked that question, you're really going to like this...

Many poisons just need to be introduced into the target's system, whether by ingestion or through a wound. So, how would you rule, in the same situation, but instead of using something as large and noticeable as a light weapon, the attacker uses a pin tipped with said poison, and uses Mosquito's Bite to pierce the skin and apply the poison?

For the record, I believe you about there not being any facing in the system. But that strikes me as odd, if not outright dumb, since only having eyes on the front of your head, you wouldn't be able to see somebody approaching you from behind.

In how many directions can you look over the course of 6 seconds? As for me, I'd say I can look in pretty much all of them.