rpm -i apt-0.5.15cnc6-4.1.fc3.rf.i386.rpm
apt-get update
apt-get -s upgrade
apt-get upgrade
This said it wanted to remove postfix and install sendmail, so I
cancelled. Then I added "postfix" to the "HoldPkgs" section of
/etc/apt/apt.conf and ran it again. It updated 4
packages. Cool.
On a Debian system, to find out what files are part of a package, do dpkg -L package-name.