If you are interested in improving your skills with Apache's mod_rewrite module, there's a great tutorial to be found here: http://www.askapache.com/htaccess/crazy-advanced-mod_rewrite-tutorial.html. Even if you are an expert already, it's definitely worth reading. One of the most interesting things in this article is the list of variables that are available when you are writing expressions in mod_rewrite. He gives a very complete list, it's even more complete than the mod_rewrite documentation. For reference, here's the list:

API_VERSION
AUTH_TYPE
CONTENT_LENGTH
CONTENT_TYPE
DOCUMENT_ROOT
GATEWAY_INTERFACE
HTTPS
HTTP_ACCEPT
HTTP_ACCEPT_CHARSET
HTTP_ACCEPT_ENCODING
HTTP_ACCEPT_LANGUAGE
HTTP_CACHE_CONTROL
HTTP_CONNECTION
HTTP_COOKIE
HTTP_FORWARDED
HTTP_HOST
HTTP_KEEP_ALIVE
HTTP_PROXY_CONNECTION
HTTP_REFERER
HTTP_USER_AGENT
IS_SUBREQ
ORIG_PATH_INFO
ORIG_PATH_TRANSLATED
ORIG_SCRIPT_FILENAME
ORIG_SCRIPT_NAME
PATH
PATH_INFO
PHP_SELF
QUERY_STRING
REDIRECT_QUERY_STRING
REDIRECT_REMOTE_USER
REDIRECT_STATUS
REDIRECT_URL
REMOTE_ADDR
REMOTE_HOST
REMOTE_IDENT
REMOTE_PORT
REMOTE_USER
REQUEST_FILENAME
REQUEST_METHOD
REQUEST_TIME
REQUEST_URI
SCRIPT_FILENAME
SCRIPT_GROUP
SCRIPT_NAME
SCRIPT_URI
SCRIPT_URL
SCRIPT_USER
SERVER_ADDR
SERVER_ADMIN
SERVER_NAME
SERVER_PORT
SERVER_PROTOCOL
SERVER_SIGNATURE
SERVER_SOFTWARE
THE_REQUEST
TIME
TIME_DAY
TIME_HOUR
TIME_MIN
TIME_MON
TIME_SEC
TIME_WDAY
TIME_YEAR
TZ
UNIQUE_ID