This tip applies only to Mac OS X Server versions 10.0.3 or 10.0.4. It does not affect the consumer OS X release.
There's a bug in the Server release which prevents the Apache web server from receiving any data sent via a CGI using the HTTP POST method of data submission. Since nearly all web-based forms use a POST submission method, this is a fairly major glitch.
Apple has published a TIL article that details a surprisingly easy workaround - you need to start and stop the Apache webserver from the terminal, not with the GUI tool. Use sudo apachectl start and sudo apachectl stop from the terminal, and disable the automatic launch of the webserver in the Server Admin GUI.
I'm assuming Apple will squash this one in 10.1, but for now, at least they've posted a solution.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20010719124357212