Post-3.0, this sequence would be Autoshot, Steady Shot, Aimed Shot, (Aimed now replaces Multi, due to it doing the same damage and being subject to the same improvements from Marks talents, but using marginally less mana) Auto, Steady, Arcane, Auto, Steady, Auto, Steady. If you're using a Noxious/Imp Stings build, you will want to fire Serpent at the start of this rotation as well.
The above, of course, depends on whether or not you've specced for Aimed Shot. At the moment I haven't, but I'm not missing it; I use Multi, especially considering that the groups I'm in generally don't like CC any more anywayz. Also, substitute Explosive Shot for Arcane above, if you've specced for it and don't have points in Imp Arcane.
The above rotation is also a rough guide; depending on how high your mana is, squeeze as many Steadies into the other shot cooldowns as you feel like. Sometimes I'll only use two, and autos, as it says up there, if my mana is low or I need to watch threat a little. Other times I'll just spam Steady and get 4 in during the time it takes for Multi to come back up.
Some people will tell you, that with the changes to autoshot mechanics in WoTLK, that the old shot rotations should not be used.
However, I still use the 1:1.5 rotation myself, and recommend it for other people as well. The single reason why is mana efficiency.
Given that the critical strike damage of autoshot has apparently been nerfed according to the forum, (although Mortal Shots' tooltip does not state this) as well as the above mentioned changes, many forum Hunters simply recommend spamming Steady Shot, with no use of autoshot between Steady.
While this may now be fine in a raid, given the new abilities of various classes to regenerate mana in raids, there will be many scenarios (soloing, primarily) where sufficiently rapid mana regeneration still will not be possible. Because of that, allowing autoshots between Steady Shots will cause your mana to go further than it would otherwise.
You also do not want to use the new Aspect of the Viper at all if you can possibly avoid it, since the 45% hit to your damage while it is up is terrible. I have lots of mana pots, and a few Dark Runes on me as well for guild 5 mans, although I admit that I probably wouldn't expend those resources for pugs. Use each one about halfway through the CD of the other, and you'll hopefully find that even if you're not at 100%, you still always have just enough mana to keep you going without Viper.
EDIT:- There's one other reason for this. I just read about people with Survival using Steady, Steady, (special), but I realised that with the long cast time of Steady, using Steady, (special; Arcane, Aimed, Multi, or ES) ala the 1:1.5, still means that you've got three shots in the air at once, which is still going to mean more damage.
Getting Lock and Load procs
Those of us who have specced for Lock and Load may well want to try and get a trap of some sort down on each pull, in order to get a proc for it. This is certainly worthwhile if you can, but I tend to find that I end up jumping in front of the tank if I try and do it, placing the trap, and then starting to shoot. That of course more or less entirely nullifies whatever extra damage you might have got from the three Explosives or Arcanes, because the amount of time you had to spend with it.
Definitely keep Serpent Sting up, however, as you have a 6% chance to proc Lock and Load from that. I don't put Serpent up at the start of a rotation, but wait until I've fired both Multi/Aimed and Arcane/ES. The reason why is because with them on CD, spending a cooldown on Serpent instead of a single Steady is going to cost me less damage than spending it instead of one of my other specials.
The other proc you want to watch out for very carefully is Kill Shot. It's the new Kill Command, and it's a ton better than the old one. You want every single proc ideally, since I anyway can get up to 5k with this; it'll give you a good spike on the meter. Hit it as soon as it comes up, and do not wait for the next GCD, as very often the mob in question will be dead by then.
Also watch your target's health, and when you're getting close to 20%, don't be afraid to fall back to straight auto for a couple of shots in order to give Kill Shot your undivided concentration; this way you won't be spending any GCDs while it comes up, and can make sure it will go off as soon as it is available.