-U Attempt to auto upgrade files that have not been user modified.
Which is a very useful option to use if you are upgrading a FreeBSD server from a -RELEASE version to a -STABLE version, for example from 8.0-RELEASE to 8-STABLE.
In such case there are a lot of files that will be modified during the installation (like those in /etc/defaults or /etc/periodic) but whose only modification is a timestamp in the cvs information at the beginning of the file.
Passing the -U argument to mergemaster will avoid you having to check each file separately.
(Lucky me I did found this!)