$ echo "Today is XX" | sed -e "s/XX/$(date +%F)/"
Today is 2012-10-11
	
s/XX/Thursday/
	
$ echo "Today is XX" | sed -f replacements.sed
Today is Thursday
	
s/XX/$(date +%F)/
	
$ echo "Today is XX" | sed -f replacements.sed
Today is $(date +%F)
	
cat <<! > replacements.sed
/XX/{s//'"$(date +%F)"'/;s/.*/echo '&'/e}
!
echo "Today is XX" | sed -f replacements.sed
	
cat <<! > replacements.sed
/XX/{
    s//'"$(date +%F)"'/
    s/.*/echo '&'/
}
!
echo "Today is XX" | sed -f replacements.sed | sh
	
aaa
bbb
ccc
	
#!/bin/sh

STRING="${1////\/}"   # using parameter expansion to prevent / collisions

shift

sed "
s/aaa/$STRING/
" "$@"
	
./sed.sh "fo/obar" <file path>
	
fo/obar
bbb
ccc
	
[ghoti@pc ~/tmp]$ cat scr.sed 
1{;h;d;};/^--$/g
[ghoti@pc ~/tmp]$ sed -f scr.sed <(date '+%Y-%m-%d'; printf 'foon--nbarn')
foo
2012-10-10
bar
[ghoti@pc ~/tmp]$