I think you are referring to vfork vs. fork. While not terribly expensive, forking certainly is more expensive than the alternatives. (There is a reason why none of the Apache MPM's do that unless you give them a very messed up config.)
However, you are right that the real issue isn't the cost of the fork (which, duh, if it were Apache would totally have you covered!!). It's more about the address space used up by each process/thread. That becomes a limiting factor for high levels of concurrency, though below levels where it may be a limiting factor the model tends to execute very efficiently (arguably more efficiently).
Apache's "event" MPM isn't really quite the same as engines light nginx & lighttpd... it works quite well, but it even describes itself as a "hybrid multi-process/mutli-threaded server".
As for sites "propagating" this information, there is certainly the ol' traditional: http://www.kegel.com/c10k.html
You can also fight some pretty well established companies in the web hosting business that really ought to know of what they speak, like say DreamHost: http://wiki.dreamhost.com/Web_Server_Performance_Comparison