Another evolution of GRES, now handling multiple files at once, and even sexier,
because its a nice single command on the line.
(foo & bar are of course your 'old pattern' and 'new pattern', while "*" is your file list (( *.html or what have you)) )
With out further ado:

root@shaolin# for i in * ; do sed s/foo/bar/g < $i > $i.bak && mv $i ; done


ta-daa!