Given two OS X keychains, create two 2-column files. First column is last modification date of an entry, the second column is the name of the entry.

gistfile1.sh

if [ "$#" -ne 2 ]; then
    echo "Usage: compare-keychains.sh first.keychain second.keychain"
    exit 1
fi

# Takes a keychain ($1), and generates a file that has a column of last-modifed dates,
# and a column of the names of items in the keychain.
# Result is written into a temp file whose name is stored in "$2".
extract() {
  local BASE=`basename "$1"`
  local NAMES_FILENAME=`mktemp -t $BASE.names`
  local DATES_FILENAME=`mktemp -t $BASE.dates`
  local MERGED_FILENAME=`mktemp -t $BASE`
  security dump-keychain "$1" | grep 0x00000007 | cut -c23- > "$NAMES_FILENAME"
  security dump-keychain "$1" | grep '"mdat"' | cut -c59-72 > "$DATES_FILENAME"
  pr -m -t "$DATES_FILENAME" "$NAMES_FILENAME" | sort > "$MERGED_FILENAME"
  rm "$DATES_FILENAME" "$NAMES_FILENAME"
  eval "$2=$MERGED_FILENAME"
}

extract $1 MERGED1
extract $2 MERGED2
meld "$MERGED1" "$MERGED2"
rm "$MERGED1" "$MERGED2"

E.g.,
20130701020437 "bla"
20130701020437 "bla"
20130701020437 "bla"