I wanted to point out that Zone of Truth is not entirely a clear-cut world breaker:
1) It allows a will save. (And do you know if the save was successful?)
2) You can still evade telling the truth by evading questions with clever or misleading answers (they only have to be true, they don't have to be transparent or to the point).
Of course, an unsophisticated target who has a low will save is in trouble, and if a cabal has any such members, they will be easy to expose.
The Zone of Truth would be coupled with a Detect Magic spell (a 1st level spell). First, the Detect Magic spell is used to determine if the subject is already under some magical protection. If so, you remove that protection. Second takes three rounds to determine if someone is affected by a charm. If the person saves, you cast it again, until he doesn't save. It doesn't take much to ensure the Zone of Truth is working.
The ability to evade with misleading answers shoudl be a nonissue. Any half-trained attorney or merchant should be shrewd enough to ensure a precise answer, usually with yes/no questions. If the person begins talking in flowery obfuscatory language, the magistrate will tell him to be more direct, or he'll be deemed to have made an admission of guilt (or, in a business context, the deal is off).
If you are concerned that the Zone of Truth caster is corrupt, I don't see how that's any different than being concerned about a corrupt judge. For suspicious people. you hire a different caster with Spellcraft to cast Detect Magic to ensure the reliability of the first. Now you have to bribe two casters whose likelihood depends in part on people believing them to be trustworthy.
While the corruption is a decent plot point, the Zone of Truth/Detect Magic combo still should have wide-ranging implications for any legal system and/or mercantile system.
This isn't even getting to the issue with Wall of Iron. Wall of Iron costs 50 gp to cast but at the earliest level (13th), creates at least 3,744 pounds of iron worth 374.4 gp. So that's a 324.4 gp profit. Every decent-sized kingdom should have at least one 13th level wizard, who can supply any kingdom with all of its iron needs casting this spell once a week (100 tons of iron//year is a fine haul for any medieval-ish campaign). Producing almost two tons of iron every day, should obviate the need for iron mines. Throw in a fabricate spell to divide the wall into easily transportable chunks.
With a simple feat, that wizard can also create a lyre of building at a cost of 6,500 gp (affordable after only 18 walls of iron!) In the hands of any competent bard (and the king should put this in the hands of his highest level servant trained in Perform (stringed instrument)) (let's say 6th level, with a +3 Charisma bonus and maxed ranks, for a bonus of +12), should manage to play on average of 4 hours. That means 1200 man-days of labor every day, at a value of (1 sp/day for untrained labor) 120 gp every day (360 gp if you consider masons a trained labor force, which they probably are). In two months (of 18 days), that lyre has paid for itself. Moreover, you just put 1200 masons out of work. That's probably every mason in the kingdom.
These are a handful of examples of how the published rules create established (and hidden) problems.