I posted on this subject on page two, and I some of the assertions I made were a little off. Take it as a good example of not researching your post before you make it. Sorry
I took a little time to look over the relevent sections, and have some observations:
1) The search skill does not have a try again listing. This means you get ONE chance to search for something. IMO you should be allowed to search again if you have new information or the situation has changed enough to allow a new try. Example: Thief says, "The door's not trapped." Fighter steps up and turns the knob. Poisoned darts fly out and hit the fighter. Thief says, "Oops! Let me look again!"
2) Taking twenty requires that the person performing the check is under "no threats or distractions" (PHB pg 65) and that "the skill being attempted carries no penalties for failure, ..." (ditto). This seems pretty clear, but every GM needs to think about the following:
a) Could you ever call a dungeon enviroment free from threats or distractions? What with monsters potentially around the next corner, and the posibility that the next floor tile is trapped and you may not find it? I think that anyone would be a little roughed up about these things, and concentration may be a little off.
b) While there is no penalty for failing a search check, there is a penalty for not finding one. If I fail to find a trap in the next 5ft square, my failure has no immediate consequences. But when I take my next step...

This goes to the explict/implict nature of the rules. There is no explicit penalty for failing a search check, but there is a pretty big implicit one. This may not be enough to qualify as a penalty for failing under the rules, but, IMO, may go toward 2a above as a distraction/threat.
These rule considerations aside, I have a dilike of allowing thieves to take 20 on something I feel is a dangerous job. I understand that finding traps can be difficult, but I have some suggestions to lessen the difficulty without resorting to taking 20.
1) Most traps in a living dungeon will have been set off at some point in the past. Wandering monsters and opposing factions in the dungeon would have set the traps off a couple of times before learning to avoid the area. This will probably have left some telltale signs, especially in the case of the more dangerous magical ones. Blacken walls point to a fire or electrical trap, a lack of dust on the floor is a good sign of a pit trap, etc. I give thieves a spot check to see if they get a 'weird feeling' about an area, with the DC determined by the obviousness of effects of a previously triggered trap. Forewarned is forearmed.
2) Circumstance bonuses. This is a bit of a house rule, but only because I disagree with the GM being able to only provide a bonus of +2 or lowering the DC by 2. I ask the thief how he will go about search the area for traps. How much time will he take? Will he take special precautions? For each thing the thief does that would lead to locating the trap, I give a +2 bonus to the roll. This gets more toward roleplaying and away from roll-playing.
Example 1:
Thief: I'm going to search the door for traps.
GM: How are you going about it. (DC for trap is 23)
Thief: I'm just going to roll. (search of 10)
GM: OK. (Rolls a 10. adds bonus for a 20) Nope, nothing.
Example 2:
Thief: I'm going to search the door for traps.
GM: How are you going about it? (DC for trap is 23)
Thief: I'm pretty sure the door is trapped, so I'm going to take my time. Say a full minute? (+2 bonus for taking time) And since we saw scorch marks in the hallway, I'm going to sniff the doorway for the smell fo brimstone fo something, and if its electrical, I'm going to pass the back of my hand over the doorway and watch for the hairs on the back of my hand to stand up. (+2 for using a smart approach with evidence at hand). I have a search of +10.
GM: OK. (rolls a 10. adds bonuses for a 24) You smell the distinct odor of achelchemist's fire coming from a series of hidden holes in the doorframe. The knob seems to have an extra catch built in that appears to activate the trap.
This gives players an incentive for getting into their role in the game.
I hope this gives GMs something to think about, and, I hope, some illumination on the subject.
