Bug#513625: aptitude upgrade action described incorrectly for the version of aptitude that will be installed
The lenny release notes say
# aptitude upgrade
>This has the effect of upgrading those packages which can be upgraded
without requiring any other packages to be removed or >installed.
However (assuming the user has followed the instructions) they will be
running lenny's aptitude which will install new packages (but won't
remove packages) when doing upgrade.