Look carefully at your includes - perhaps you have a blank line after a closing ?> ?
This will cause some literal whitespace to be sent as output, preventing you from making subsequent header calls.
Note that it is legal to leave the close ?> off the include file, which is a useful idiom for avoiding this problem.
(EDIT: looking at your header, you need to avoid doing any HTML output if you want to output headers, or use output buffering to capture it).
Finally, as the PHP manual page for header points out, you should really use full URLs to redirect:
Note: HTTP/1.1 requires an absolute
URI as argument to Location:
including the scheme, hostname and
absolute path, but some clients accept
relative URIs. You can usually use
$_SERVER['HTTP_HOST'],
$_SERVER['PHP_SELF'] and dirname() to
make an absolute URI from a relative
one yourself:
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