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Then how, pray tell, do you explain using Stealth checks out of combat, at all?  Nothing in the Stealth check rules have anything to do with anything other than becoming Hidden, and then the Rules of Hidden Club.

You might note I said out of combat, not out of encounter.  An encounter that involves combat is, in fact, a combat encounter.  You'll also note that I, too, said that rolling a stealth check over and over is wrong.  The difference is that rolling a check, once, is perfectly acceptable and in fact a good idea.  How else to you explain the other side, when it's the PCs encountering traps?  Their perception is being checked against a DC that the DM just made up, practically, but mechanically it's being checked against a Stealth check result from some actor previous to the party showing up.  And it could be millenia before the party shows up.  Yet you would have me believe that a stealth check has to be rolled when the party arrives, to have something to check against the PCs' perception?  I don't think so.

The trick with setting up any ambush is that its only as good as the least stealthy party member (just like you don't have to outrun the dragon, you only have to outrun the dwarf or halfling).  It doesn't really matter if your thief rolls a 42 if the Paladin in plate armor rolls a 12.


I would run the setup of an ambush as a Skill Challenge.  The complexity level would vary based on what you're trying to surprise and where.   If its a single creature with no special detection abilities (ex: a lone merchant in a dark alleyway with no one else around) it would be complexity 1. If it's a caravan with trained guards (or an army) including darkvision and flying scouts on an open road in a barren wasteland, and your actual target is at the rear end of the caravan, it's complexity 5.
 
  Relevant skills would be:
1) Stealth (obviously). This would be the main skill
2) Nature, Dungeoneering or Streetwise, based on location of the encounter.  I would limit the number of these checks, or after the 1st one or two, it might become advantage granting only.
3) Perception and possibly Insight could give advantages - spotting better hiding locations, etc.
4) Bluff or Thievery are possibilities, especially to setup another distraction, or if the ambush relies on a character faking they are injured in the road, etc.  Or it could represent the party having a hidden signal to trigger the attack without alerting the enemy.
5) Acrobatics and Athletics could  play a part as well with a limited number of uses and represent moving something large into position to give cover (and not look obvious), or a character getting into a hidden position up in a tree, etc. 
6) Endurance may also come into play.  Sitting in ambush position may be easy for a character trained in stealth and in light armor.  Its more difficult for the paladin in plate or cleric in scale armor to sit still quietly for those 5 minutes you're waiting.
7) If the nature of the approaching targets is known, I would maybe grant an advantage from a monster knowledge check of the appropriate skill.

Challenge results:
On total success, I would give the full party a surprise round, and starting as hidden, or at least a very large bonus (say +20) on the opposed stealth check as the enemy approaches.

On failure, the enemy would recognize there is an ambush or something wrong as they approach the area and would be alert, but the party would still get stealth checks without a bonus (I don't think a penalty would be necessary).  If there's time, I may allow the party to recognize they totally screwed up their ambush attempt ahead of time, but they wouldn't be able to retry the challenge near the same location, and if they move further down the road, a reattempt may have penalties or be more difficult as the enemy would be alerted.

If the result is somewhere in between (challenge succeeded with some failures), grant a moderate bonus to stealth checks to the party.  Considering lines of sight based on the enemy and the party members, this might result in a surprise round or not, depending on how good the enemy's perception is.
..."window.parent.tinyMCE.get('post_content').onLoad.dispatch();" contenteditable="true" />  Also consider that depending on terrain some characters may need to take positions that are not great for a surprise round in order to be hidden, such as being prone, up in a tree, in difficult terrain (heavy brush) etc, so some characters may choose not to act in the surprise round if they haven't been spotted.

I can tell you how I am handling abmush/stealth rules now, as I discussed with the players involved in my campaign. It follows all the rules and works well IMO.

Players can hide all they want if there is no chance of enemy detection. No dice rolls. You want to hide, fine, you are now hidden. (This allows players to set up an ambush in an area and then only have to maintain normal cover or normal concealment, because they are just having to remain hidden from that point on.)

The first dice roll occurs when an enemy has its first chance at detecting any player. For example, a caravan first comes down the road. I consider this the start of the encounter, although combat has not yet started so a noncombat enounter. The caravan is most likely over 10 squares away, so the caravan folks have a -2 to their perception at this point. The players roll their stealth check. If any of the players fail, they obviously are detected and probably combat starts with no surprise round. If they instead all succeed at their stealth checks, any of them can reroll their stealth check if they choose to, following the rule of rolling another stealth check after a "move 0" action. They have to keep their new roll and could possibly be detected if they roll low and as the caravan draws nearer. And I calculate how much closer the caravan is each "round" although official rounds have not really started yet since we are not in combat. This limits how many rerolls they are going to get before the risk becomes too great. They are probably only going to get 1-2 lower risk rerolls before the caravan no longer has the -2 perception check due to being over 10 squares away.

Does this sound reasonable? Trying to incorporate the information you all are providing for a final ruling for my players. I personally don't like the skill challenge concept for something that really should just be covered under the stealth skill.