You want to upgrade your FreeBSD system before 9.0 through freebsd-update and it fails, showing the following error message:
The update metadata is correctly signed, but failed an integrity check. Cowardly refusing to proceed any further.
The problem lies in a broken version of freebsd-update(8) which got fixed in FreeBSD 9.0-RC1.
To fix your version simply run the following command:
bsdbox# sed -i '' -e 's/=_/=%@_/' /usr/sbin/freebsd-update
For a full reference read the official FreeBSD 9.0-RC1 announcement.