One-line global string replace in all files with sed (or awk?)

[Anthony Atkielski]

A few years ago, I'm sure I came across a one-line way of replacing
every occurence of one string with another in an entire directory of
files (potentially including all subdirectories as well).  I think it
used sed or awk.  Now I can't find it.  The examples on the Web are all
multiline scripts or programs, but I'm sure I saw a way to do it all on
just one line.

Can anyone tell me how to do this?
-- 
Anthony