Using rewrite_header

I'm a little confused about something I read:

http://marc.merlins.org/linux/exim/files/sa-exim-4.2.1/README

Under "CONFIGURING SPAMASSASSIN" it says:

"Since SA is usually configured to pass messages on that are beyond  
the SA
spam threshold, it can make sense to rewrite the subject line."

I have told SA to rewrite my subject:

rewrite_header Subject ***** _HOSTNAME_ thinks this is spam (_SCORE_/ 
_REQD_) *****

But the subject doesn't seem to be rewritten when a message is marked  
as spam:

<snip>
X-First-Run-SpamCheck: Checked on SMTP Transfer
X-First-Run-SpamScore: 4.7
X-First-Run-is-Spam: Yes
X-SA-Exim-Connect-IP: 216.7.194.254
X-SA-Exim-Mail-From: [email protected]
Subject: product for you... but i think u need to buy it
X-Spam-Flag: YES
X-Spam-Checker-Version: SpamAssassin 3.1.3 (2006-06-01) on
         ricky.example.ca
X-Spam-Level: ****
X-Spam-Status: Yes, score=4.7 required=4.0 tests=DRUGS_ERECTILE,
         MISSING_HEADERS,MISSING_MIMEOLE,NO_PRESCRIPTION,TO_CC_NONE,
         UNPARSEABLE_RELAY autolearn=no version=3.1.3
Content-Type: multipart/mixed; boundary="----------=_44C46B44.F0D18FEE"
X-SA-Exim-Version: 4.2
X-SA-Exim-Scanned: Yes (on mail.example.ca)
</snip>

Am I missing something somewhere?