Would it be a faux pas for a DM to also play a PC in the same game? I suppose it could garner the DMs PC an advantage.
People here don't like it. People here complain about a lot of social problems in D&D that I've never seen in real life (from smelly gamers to killer DM's to players who tell other players what to do). Like Steven Colbert talking about race, I assume it's real because people tell me about it, but I don't see it.
My experience with the DMPC is that it's fine.
The first campaign I played in was just the DM, his little brother, and me -- us two players never having played before. It made sense for the DM to create and manage some of the characters, and we never did file a union grievance about DMPC's takin' our jobs. So it always seemed natural to me.
The main complaints seem to be:
1) DMPC's are unfair. DM's will use their inside knowledge of the campaign and overwhelming stat and treasure advantages to boss us around and do all the cool stuff. Their characters will be super strong and we'll just be BMX Bandits to the DM's Angel Summoner, or Pippin and Merry to his Gandalf.
The way to solve this, shockingly, is for the DM to be a good DM. If he can design balanced NPCs in the campaign setting, and role play them with in-character knowledge only, he should be able to run an NPC party member.
If a DM can't handle running one party-member NPC that way, how can he "be trusted" to run the non-party NPC's and monster?
Surely the bartender in the tavern where your party just met each other is an NPC -- but he's run by the DM, so he might be the dreaded DMPC! Maybe the DM is planning to have him boss you around and be super powerful -- you'd better just tear up your character sheet and stomp off at the first sign of an NPC who might have a NAME -- too dangerous!
The other issue here is in how you treat the game. I've always seen D&D as a cooperative game where the team is working together to defeat a challenge. It doesn't make me mad when another player does something successfully, and I don't feel any different about an NPC on our side "scoring a point for the team". Others get upset if there's any "imbalance" between characters, or they get "upstaged" in any way.
2) The DM is overworked and doesn't have time to run a character.
With more complicated sytems (3e and 4e at higher levels), there's a good point here. The DM can design a DMPC or NPC party member, but he'll need to hand them off during play to not be distracted.
This is more true with a big group, where an NPC party member is less likely to be needed anyhow. For a tiny group (1 or 2 players), it may or may not make sense for the DM to actively run the character -- maybe role play out of combat but hand over in combat.
So back to the original question -- yes, DMPC's are a Faux Pas here, like wearing white after Labor Day. But that doesn't you can't do it and enjoy it.