>> What happens when John leaves the company, and he's trained everyone
 >> to, whenever a certain class of error happens, go to him without
 >> learning anything about the problem first?

 That does concern me... It makes economic sense to me the rule "If a
 person is indispensable to your company, fire him now, since it
 will cost you more when you do eventually loose him."

 Thus I have several counters to the Truck Factor...
 1) Unit Test / TDD === executable documentation.
 2) Good rdocs that I periodically refresh.
 3) Get others to code review my changes.
 4) Go on holiday. The "It's Johns fault" message tells them immediately
     they had better start reading code if they want it fixed before I get back...