I found this on a Website (
www.checkupdown.com)
Resolving 501 errors - general
This error should be very rare in any Web browser. It is more likely if the client is not a Web browser - particularly if the Web server is old. In either case if the client has specified a valid request type, then the Web server is either responding incorrectly or simply needs to be upgraded.
Are you sure apache supports HTTP1.1? Or is that a script that generates the error?
if you have a recent (stable) apache with http1.0 and http1.1 support and it is a common html file that causes problems, then.... well yes... cant tell ya more then.
g
raven