If there's nothing in the server logs, then maybe the response is actually being sent but your client isn't interpreting it correctly.

In general, when I've got a mystery of the form "this is a simple response, why isn't the browser reading it right?", I've found lwp-request invaluable, particularly the "-e" flag for looking at all the headers. Firebug is what all the cool kids are using for inspecting HTTP requests these days, but I like calling it from the command line.

I know you skipped some of the conditions in the sample, but maybe you forgot a "Status" line? You might check http://httpd.apache.org/docs/2.0/custom-error.html#custom to see whether the comment about a location having no effect is relevant.