I've seen a few in 4e so far. One happened because the group was ill-prepared for the fight, and played right into the monster's strengths, and never changed tactics even though they knew they were fighting the creature sub-optimally (needlefang drake swarm, for what it's worth). I wound resetting the encounter, and the party ran it again, and while it was tough, they did better once they changed their tactic.
With a similar party, they were TPKed by a low level solo brute I made up. It was another case of the party playing right into the creature's strengths, coupled with aiming at hard to hit defences and not fighting as a party (ie, no tactics, just "rush it and kill it", which was the usual 3e tactic).
In the 3rd 4e TPK I've been in, I was a player, and it was mostly a case of the PCs being unable to avoid the encounter and having already gone through about nine encounters that day (we were low level). I think the players in that encounter were a bit tired of having encounters thrown at us, using metagame reasoning that if we ran from this one, we'd just get hit by another (which was probably untrue) and we fought it. And got destroyed.
A few early warning signs of a TPK I've noticed in 4e:
1) The monsters are hitting, and the PCs are not. If this is happening a lot, there is usually a reason this is happening. If it's the fact that GMs monsters are higher level, this could be a problem. If it's just that the PCs aren't functioning as a unit, and the GM's monsters are, then as a GM, just let the encounter carry on (and maybe point out that the PCs should be working together).
2) If the party is really low on surges, a TPK is almost certainly a possibility. If you're on the player's side of the screen, try to run. If you can't run, the GM is gunning for you, and accept your death bravely. IF you're the GM, either call of the fight, or let the PCs run. PCs should almost always have a way to avoid a fight if they're low on surges... unless they're the one that started it, in which case... bring it on.
3) The PCs keep using the same tactic, and it doesn't work this time around. If this happens, it can be a good warning sign - the PCs are using the same old tactical approach, and it just doesn't work on this particular combo of monsters. As an example, a mostly-ranged party is fighting a monster that is resistant to ranged attacks, or on a battlefield with a lot of concealing terrain - and they still focus on ranged attacks while the monsters tear the party apart. Also applies when a "rush one monster at a time" party gets divided, and doesn't know how to cope with the new situation. My advice? Let the encounter ride - it's a party problem, not yours.
4) If the PCs are using only at-wills (having used all their dailies and encounters) and the monsters are still using encounters, it can be a good warning sign that bad things are around the corner.
5) The GM plays monsters to their best abilities... and the players refuse to use daily powers. This combination will eventually lead to a TPK. Also, if PCs rarely use dailies in the first few rounds, it can lead to TPKs in a lot of cases (Dailies are best spent early, after all).