Try bufdo, argdo, windo, and tabdo depending on the subset of files you'd like to modify. E.g., to replace "foo" with "bar" in all open buffers:

  :bufdo %s/foo/bar/g | update

Although I wouldn't knock `bash command madness' for this sort of task. :)

The Unix shell is pretty much designed for advanced text manipulation across files. E.g., to replace all instances of foo with bar in HTML files, recursively:

  $ find . -name '*.html' -exec perl -i -lpe 's/foo/bar/g' {} \;

vim and the shell are not so far apart and complement each other well, so it pays to understand them both.