This was discussed long time ago and patch was quite big. Two points were discussed:
1) Portability of <cheader> headers; need to test with g++-3.4.6 and g++-4.0.
2) Sorting order of includes: it would be good if we can standardize an order, e.g. first local module headers, then ns3/ headers, then C++ headers and C headers, and finally Unix headers if any.

I will post a cleaned-up patch that applies against current codebase in a few days.

include-what-you-use[1] could also be helpful here, but in that case we should probably first make ns-3 build with clang since it's clang-based.

[1] http://code.google.com/p/include-what-you-use/