"... if you need more than 3 levels of indentation, you're screwed anyway, and should fix your program." - Linus Torvalds, Linux kernel coding style (partly justifying why the Linux kernel style uses 8-space indentation)
I was thinking of that quote too. And with Python-style OOP languages, I tend to increase that to 4 or 5... However, I never really ran into a problem with it until I started doing stuff in NodeJS and entered callback hell. The excellent Futures and Async libraries have helped reduce a lot of nested indentation, much more than getting rid of lambdas does. (And getting rid of lambdas has its own drawbacks.)
http://www.kernel.org/doc/Documentation/CodingStyle