Here is a little patch to forward www-data mail to root by default
Index: debian/exim4-config.postinst
===================================================================
--- debian/exim4-config.postinst (revision 2305)
+++ debian/exim4-config.postinst (working copy)
@@ -60,7 +60,7 @@
writealiases() {
echo '# /etc/aliases' > /etc/aliases.tmp
echo 'mailer-daemon: postmaster' >> /etc/aliases.tmp
-for i in postmaster nobody hostmaster usenet news webmaster www ftp abuse noc security ; do
+for i in postmaster nobody hostmaster usenet news webmaster www-data ftp abuse noc security ; do
echo "${i}: root"
done >> /etc/aliases.tmp
mv /etc/aliases.tmp /etc/aliases